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Since my first study posted in August I have learned a lot more about guns. I also found out about a number of errors I made. Those need to be corrected. I’m still far from regarding myself of beeing an expert. So anybody who finds an error that has led me in a wrong direction please feel free to correct me. I try to keep this study free from to much military technical terms. Well, mostly. A nice glossary for terms and abreviations can be found here. http://www.inetres.com/gp/military/glossary/index.html
Of course, I don't think I'm able to tell what guns might be in 500 years. All I can try to find out what they would be alike if they where made today.
Still there are good reasons to pedict that they will be alike in 500 years. There will be improvements concernig weight, accuracy, rate of fire, aiming and stuff like that but they will not change in principle. A barrel will be a barrel no matter if it is made of steel, ceramic carbon composites or something more strange than what can be imaginated today. Same for breech, breechblock and cradle.
First of all there are the laws of physics (especially ballistics) that will rule in 500 years too. Than there are the requirements of military logistics. Last but not least are the requierements of the battlefield. Battlefield conditions can of course change strongly but they will not leave a certain “frame” set by environment, weather and daytime. Most of the fighting will happen under conditions like we have them today because most conflicts arise from arguements over political influence on habitable places. A further lot of conflicts will be over natural resources and production capacity. So far, war is mostly nothing but big style robbery. While most of these conflicts will happen in habitable environment a lot will not. This certain number of new Battlefields will have a strong impact on decisions concerning usefullness of equipment. Current equipment can be used under harsh conditions but it generally optimised for a one g, one bar, oxygen rich and comparably modest temperated environment. Battletech equipment is generally designed to work in other environments too. So far many of the weapons we know today and who are superior to those of B-tech will probably not be a good idea and need to be enhanced. Think about temperatures below –50°C or above +40°C. Due to the human genius in developing WMDs the battle vehicles of today could fight already under conditions which might come natural on some planets like high radiation or poisenous atmospheres. These abilitys need to be expanded for a fight in low or zero-oxygen atmospheres too. Actually I can’t see why somebody wants to fight in one high pressure, hot and corrosive atmosphere (Venus) but fighting under different gravity conditions will be quite usual.
Due to small number of “facts” I have about the different sorts of guns I had tried at first to find out what kind of ammunition (calibers) B-tech guns would probably have. From that knowledge I’ll try to find out about the guns.
In Battletech guns are devided into several groups according to their characteristics. There are Autocannons (AC’s) and LBX (Large Bore eXtended) cannons. Amazing enough, but some people out there don’t know what LBX is standing for. Details about how these guns are working are not known.
Autocannons in B-tech are sorted by the damage per "shot" they inflict on the target and not by caliber or rate of fire. That's fine for gaming but it is insufficient for writers and for RTS. The ammunition of a weapon is calculated as "shots per ton of ammo". Unfortunatly the size of one shot (rounds per shot) is unknown which means: One "shot" can be anything between a single round fired and a burst of rounds fired (no mattter how many rounds). On the other hand, that tells us at least the maximum caliber of a gun in case it fires one round per shot. Any higher number of slugs per shot will reduce the caliber accordingly.
The following study is based on the assumption that AC’s are firing full bore ammunition (full bore = barrel and slug have the same caliber). Call it backwards logic but LBX cannons are described as “shotgun like” which can fire also subcaliber slugs and that is making AC’s firing full bore ammo. Another “fact” derived from the novels is that all AC- and LBX cannon slugs in B-tech are “kinetic effect” only (Depleted Uranium is named). The rules of the board game are supporting this hypothesis further. Even the AC20 projectils don’t have any ground effect like the Long Tom artillery.
The blast and shrapnell radius of actual shells in 80mm – 100mm caliber is about 50 m. That’s around 2 hexes in Battletech. For me, one high caliber, high explosiv round that hasn’t any ground effect is beyond imagination.
Actual attempts to enforce the kinetic effect by adding explosivs as second stage propellant to gain more speed or as part of the projectil to gain a better penetration performance have just minor effects to calculations because the share in weight of an extra dose of explosivs is nil in comparison to the total weight.
A third assumption derived from novels and MW games is that all cannons are firing bursts for beeing considered as “Auto-“.
Let's start with ammunition for ACs, UACs and RACs.
What's known from the rules are the crits and weight for the guns itself and the weights of the ammo in shot per ton (toa = ton of ammo).
AC2 (IS) 6 tons (damage 2 points) 45 shots / toa.
AC5 (IS) 8 tons (damage 5 points) 20 shots / toa
AC10 (IS) 12 tons (damge 10 points) 10 shots / toa.
AC20 (IS) 14 tons (damage 20 points) 5 shots / toa.
UAC2 (Clan) 5 tons (damage 2 points) 45 shots / toa.
UAC2 (IS) 7 tons (damage is 2 per) 45 shots / toa.
UAC5 (Clan) 7 tons (damge 5 points) 20 shots / toa.
UAC5 (IS) 9 tons (damage 5 points) 20 shots / toa.
UAC10 (clan) 10 tons (damage 10 points) 10 shots / toa.
UAC10 (IS) 13 tons (damage 10 points) 10 shots / toa.
UAC20 (Clan) 12 tons (damage 20 points) 5 shots / toa.
UAC20 (IS) 15 tons (damage 20 points) 5 shots / toa.
RAC2 (Clan) 7 tons (damage 2 points) 45 shots / toa.
RAC2 (IS) 8 tons (damage 2 points) 45 shots /toa.
RAC5 (Clan) 10 tons (damage 5 points) 20 shots /toa.
RAC5 (IS) 10 tons (damage 5 points) 20 shots /toa.
RAC10 (Clan) 14 tons (damage 10 points) 10 shots / toa.
RAC20 (Clan) 16 tons (damage 20 points) 5 shots /toa.
Ammo of the same “shot size” are inflicting the same damage per shot no matter from which faction they are made. But UACs can fire two shots per turn and RACs can fire six shots per turn. Therefore they can inflict two times or six times the damage of an AC.
The keyword is "can". Because, an UAC 20 with one ton of ammo (5 shots per ton) can just fire two times before it runs out of ammo. Therefore in Classic Battletech a player can decide wether he wants to fire an UAC in AC or in UAC mode. Due to the additional risk of a jam the UAC's are usually used in AC mode. The additional firepower is restricted to targets of opportunity or to critical situations.
RACs have a higher risk of jamming. A Clan RAC20 fires six shots per turn so it needs at least two tons of ammo for firing at least one time. Having done so, the gun has 4 shots left from its two ton payload that can't be fired in RAC mode. I don't know wether there are rules for firing a RAC in AC mode. Bet is, those rules exist at least as house rules.
The slight differences ranges of IS AC,UAC and RAC are for game balancing because Clan UAC and RAC have the same range stats. Assumption is, they all use the same ammo (caliber). That’s not unlikely. Today’s artillery is using a very narrow range of calibers and modern guns are still able to fire grennades made in the fifty’s of the last century.
On a first view, guns in Battletech are most comparable with anti tank guns of today. According to Herb Beas from “Classic Battletech” Battlemechs are just walking tanks and B-tech tanks and mechs are using their guns in the same way like actual tanks. Together with the “fact” that guns of mechs and tanks in Battletech are essentially the same a comparison of B-tech guns and there ammo with tank guns of today will be first.
Any kind of projectile or warhead with the sole purpose of crushing armor is described as “armor penetrating”. Wether this is achieved by drilling through the armor with an arrow like penetrator or with the plasma blast of a shaped charge warhead or by brutally crushing on the outside with kinetic ammo or with high explosivs doesn’t matter. The use of ablativ armor in Battletech does not interfere with this concept.
So far from the three types of rounds used in modern tank guns (High Explosive, High Explosive Anti Tank and Kinetic Energy) only KE ammo is comparable. The KE cartridge of today contains one APFSDS (Armor piercing Fin stabilised Discarding Sabot) or in short one sub caliber slug. All tank guns can also fire canister rounds (Shotgun shells for tanks).
http://www.defense-update.com/products/digits/120he-mp.htm.
So far, up to level 2 all gun rounds in B-tech have a cartridge (case) holding together the primer, the propellant and the projectile (shell with fuse). The three components with the highest effect on the weight of a gun round are case, propellent and projectile
AC ammo needs to have a case. The loading of two different components needs to much time for burst weapons. In this regard russian tank autoloaders are still to slow. From Level 3 on there are also caseless ACs available for B-tech guns.
Is that odd? Modern anti tank guns like the one in the Abrams M1 and Leopard 2 are using already partly combustible casings. In this case (pardon the pun) the cartridge weight is calculated partly as propellent. The problem is that those tank guns can not be regarded as Autocanons because they are not firing bursts. There is no actual autocannon that uses partly combustable casings. I suppose, the actually available casings are not stress resistent enough for becoming handled via autoloader in burst mode. Not right now, but may be in 1000 years?
Still some data. A 120 mm KE (Kinetic energy) round DM 43 A1 for the Leopard 2 gun has a penetrator ('cause it's APFSDS) of 7.2 kg (with sabot) and 7.6 kg of propellant. From the total weight of the whole cartrige (20 kg) the case weight left is 5.2 kg.
A real modern 120 mm HE (High explosive) round for the Leopard 2 gun has a shell weight of 19 kg and 3 kg of propellant. I suppose the non combustable part will weight similarly.
In difference to that caseless ammo in B-tech is meant to be without any casing (G11 style?).
Caseless AC2 (IS) 6 tons (damge 2 points) 67 shots per ton of ammo.
Caseless AC5 (IS) 8 tons (damage 5 points) 30 shots per ton of ammo.
Caseless AC10 (IS) 12 tons (damage 10 points) 15 shots per ton of ammo.
Caseless AC20 (IS) 14 tons (damage 20 points) 8 shots per ton of ammo.
Actually the weight, damage and range stats of the guns in the same damage class are the same.
Firing caseless ammo has no effect on the gun stats. But for the caseless ammo the number of shots per ton has been increased compared with cased ammo. This gives further information about the weight of a case in relation to the weight of the total cartridge (with slug and propellant). A caseless AC20 "cartridge" has just 62.5 % of the weight of the AC20 cartridge. Other numbers are 67.2 % (AC2) and 66,6% (AC5, AC10).
Therefore the “case” weight of an AC round is arount 35 % of the total weight. Compared with 25% of today’s ammo. At the end of the 50' in last century the french army had used 105 mm rounds for their AMX 30 MBT. The HEAT round OCC-105-F1 and the HE round OE-105-F1 had those 35% of case weight ratio.
http://www.kotsch88.de/m_105mm-m57-2.htm
Back to the kinetic effect warheads. According to B-tech definition all modern tank guns above 20 mm caliber are LBX because Armor penetrating Kinetik Energy rounds of today are usualy sub caliber slugs.
I have not found any information wether barrels of B-tech guns are smooth bore (need for a fin stabilised ammo) or rifled. But there is a good reason to assume that AC’s in Battletech will have rifled barrels. Battletech AC’s have to work inside AND outside of an atmosphere. Outside of an atmosphere there is nothing to stabilise the fins of fin stabilised slugs. They need to be “spin stabilised”. An example for a spin stabilised subcaliber KE slug is the russian BM-8.
The idea of spin-stabilisation-only is further supporting the KE-only hypothesis because shaped charge HEAT is less efficient when fired from a rifled barrel (there are ways to engineer around this problem).
While the smooth bore tank guns of today are firing fin stabilised sub caliber KE slugs…
…the high caliber full-bore KE ammo of today is found usually in naval artillery.
Artillery on the other hand is usually using HE shells. Those are full bore for having a maximum volume for the payload (= Explosives). The more recent advances in aiming technology have led to a return of amor penetrating (APDS) ammo into Naval artillery. The shape of full bore, spin stabilised artillery slugs has not evolved much since the fifty’s of last century. Logic say’s that the shape will also not change in future because also a twist stabilised, full bore KE slug needs to hit precisely. And that will happen only when shape and balance of the slug allow a proper stabilised flight.
Also until very recently, gun barrels in Artillery and naval artillery have allways been rifled.
For more info about ammo and ballistics:
http://www.globalsecurity.org/military/systems/munitions/ammo.htm
A little history of KE ammo.
KE ammo became developed at first during World War I and became used by all participants in the beginning of World War II also. They were fired from specialised rifles (Panzerbüchse) like the modern BAR .50. During World War II, the USA (APCR) and Germany (PaK 40 = 75mm x 714R) had developed sabot projectils for their Tankhunters. Those slugs had a full bore, soft Sabot (french: Sabot = wooden shoe) around a tungsten core. By impacting on target the whole kinetic energy is driving the hardened core into the surface of the armor. The wider sabot can’t penetrate. It becomes stopped by the armor surface, becomes squashed or desintegrates to shrapnell and transfers most of its kinetic energy to the core. This “energy transfer” helps the core to drill right through the armor.
Today this kind of Ammo is known as APDS (Armor Piercing Discarding Sabot). A subspecies is F-APDS (Frangible Armor Piercing Discarding Sabot) used in AA guns. In land warfare it has survived in autocannon calibers up to 35 mm.
See also: http://www.quarry.nildram.co.uk/tankbusters.htm
Here I need to correct an assumption from the first study about B-tech guns.
My earlier assumption was that the share of propellant on the whole weight can be ignored. That’s wrong. Battletech AC’s and LBX cannons are not firing over obstacles. The trajectory (balistic curve) is described as very low. The way to get this done in modern ammo is by using a higher propellant weight together with a similar heavy slug. Theoretically such an ammo has a very long maximum range. But real combat ranges (effective ranges) for tank (cannon-) battles are still under 3000 m (800m at night). Over longer ranges it is to difficult to hit a moving target with an unguided projectil. Effectiv ranges in Battletech are, for what reason ever, shorter.
In modern tank KE ammo the ratio of propellant to slug (with sabot) weight is nearly 50/50.
Some data of actual kinetic effect APFSDS ammo for comparison:
30 mm Bushmaster: 0,75 kg for the whole round. Projectile (with Sabot?): 0,235 kg. Propellant: 0,176 kg. Vo 1385 m/s. Propellant to projectile weight ratio: 42.8 /57.2 (in %)
90 mm (M690A2): Developed from the earlier 90 mm M690 and M690A1, this round fires a tungsten alloy penetrator rod at a muzzle velocity of 1,345 m/s. The penetrator can defeat a NATO Heavy Target (150 mm RHA at 60  at 2,000 m or a NATO Medium Target (130 mm RHA at 60 at 3,000 m. Projectile time of flight to 1,500 m is 1.2 seconds and 1.6 seconds to 2,000 m. The apogee for 1,700 m is less than 2.3 m. Round weight (steel cartridge?) : 11.5 kg. Projectile (with Sabot?): 3.6 kg; Propellant: 3.3 kg.
The propellant to projectile ratio is close to 50/50 (47,8/52.2).
100 mm (Russian BM-20): No data found for total weight. Projectile weight: 4.58 kg (with 4.06 kg for the Penetrator). Propellant weight: 5.9 kg. Vo: 1430 m/s.
The propellant to projectile weight ratio is 56.3/43.7.
105 mm (M900): Choosen for a Stryker variant. Total weight: 18,5 kg. The DU APFSDS: 6.86 kg. Propellant: 6.12 kg. Vo: 1,505 m/s
The propellant to projectile ratio is 47.2/52.8.
115 mm (Russian 3BM21): No data for total weight. Projectile: 6.26 kg (with a 4.5 kg Tungsten Penetrator). Propellant: 8.2 kg. Vo is 1600 m/s.
The propellant to projectile (with sabot) weight ratio is 56.7/43.3.
No data are found for the more recent BM-28 with DU penetrator.
120 mm (DM 43 A1): Round weight: 20 kg. Projectile (with sabot): 7.2 kg. Propellant: 7.6 kg.
Vo: 1,740 m/s. The propellant to projectile (with sabot) weight ratio is 51,4/48.6.
125mm 3VBM13 (Russian): No data for total weight. Projectile: 7.05 kg (with a 4.85 kg DU Penetrator). Propellant: 5 kg. Vo is 1700 m/s.
125mm 3VBM17 (Russian): Total roundweight 20.4 kg. Projectile: 7.05 kg (with a 4.85 kg Tungsten Penetrator). Propellant: 5 kg. Vo is 1700 m/s.
The propellant to projectile (with sabot) weight ratio is 41.5/58.5 .
Note: Russian tanks guns are usually equipped with an autoloader. The magazin is a ring located around the turret basis.  numbers 1 & 2.
A complete round would stand up to high for becoming loaded via autoloader. For getting around this problem the ammo is devided into a projectil and a charge. Both are stored together but seperated and are joined during the loading process. 
For the 125mm D-81 gun the charges are partly combustible and have total weight of 10 kg with a 3.4 kg stub and 5 kg of propellant. http://armor.kiev.ua/fofanov/Tanks/
Actually the discardable sabots of APFSDS ammo are made of alluminium and have an unignorable share on the projectils weight. Data about the weight for most recent penetrators alone are somewhat fishy. Some sources are telling about 4,7 to 5 kg of Tungsten alloy for an older german DM 13 APFSDS.
Fortunatly, in difference to a fin stabilised discarded sabot (DM 43 A1), a full bore APDS (not fin stabilised) will hit the target with its total mass.
From the data above and with exception of the russian 125 mm calibers, it seems as if the russians prefer a higher propellant load compared with NATO ammo. So, for a start, let’s assume the 50/50 ratio for propellant and slug weight are valid for calibers above 30 mm.
According to Newton the kinetic energy of a projectil is calculated as half the mass multiplied with the square of its velocity. Therefore any additional speed is more important than additional mass.
Just one example: The DM 43A1. For simplicity reasons lets assume that the slug will hit with nearly the same speed as it’s Vo (certainly more likely if fired on just 500 m = 18 hexes). Such a 7.2 kg sabot will hit with a kinetic energy of ca. 10.9 MegaJoule.
There is another interesting observation. In all cases the muzzle velocity (Vo) is increasing with caliber. I will come back to that later.
There is still a little uncertainty left. B-tech ammo comes in a variable number of shots per ton of ammo. But will this be the brutto or the netto weight?
From the B-tech novels it seems that the rounds of one shot are held together by ammo clips. Much like the way ammo is stored on assault rifles. In case the shots per ton of ammo are netto than there is no problem. But if it is brutto than the weight of the clips have to be substracted and that will further reduce the weight of the single round and therefore also reduce its caliber. For now I will assume that the shot weight is netto and I will come back to this question later.
By now the frame is set to describe B-tech AC ammunition.
I wasn't sure which kind of ton is the basis for Whizkids but there is not so much a difference between Metric ton, US ton and UK ton so I stick to metric tons.
So far the only real high caliber ACs can be found in naval artillery. Whenever possible I tried to reference ammunition from such a gun. Unfortunately, some guns for artillery and heavy naval artillery are using variable loads of propellent for better adjusting range. Such ammo does not have a case and the the comparison is for the projectil only. Of course, such guns are not rated as ACs either. Still their example might be good enough to get an idea about the caliber and, of course, the size of the gun.
For calculating the calibers of Battletech AC’s the following factors are used:
Slug and propellant together are 67.2 % (AC2), 66.6% (AC5,AC10) and 62,5 % (AC20) of the total cartridge weight. The slug weight is always 50 % of the slug-and-propellant weight.
AC2 calculation for one round per shot: one ton = 45 shots = 45 rounds = 22.2 kg per cartridge = 14,9 kg per slug/propellent = 7,45 kg for the slug.
There is no known caliber with a matching warhead. It needs to be above above the 76 mm /62 Compact (US/Italy/Japan) with a slug weight up to 6.6 kg and below the the UK 3.7 inch Howitzer 9.1 kg slug in 94 mm caliber.
Thinking about the AC2 as an burst firing gun. Calculated for a 5 rounds burst: 4,44 kg per cartridge and 2,98 kg per slug/propellant = 1.49 kg per slug.
Best matching caliber is the outdated Russian navy 45 mm/78 (1.77") SM-7 from 1950 with 1.41 kg per slug. The weapon had a rate of fire of 135-160 rounds per minute. Next best is the Bofors 40 mm/70 Mark3 with ca. 1 kg.
AC5 calculated for one round per shot: one ton = 20 shots = 20 rounds = 50 kg per cartridge = 33 kg per slug/propellant = 16,5 kg per slug.
Best match is the Russian 100 mm/70 (AK-100) with a warhead of 15.6 kg next best would be the GER/FRA 100 mm/55 Model 1964 with 13.5 kg per slug
Thinking about the AC5 as a burst firing gun. Calculated for a 5 slug burst: 10 kg per cartridge and 6,66 kg per slug/propellant = 3,33 kg per slug.
There is no known caliber with a matching warhead. It needs to be above the US 57mm/70 Mark110 with weights up to 2.8 kg but well below the RUS 76mm/59 AK-176 with 5.9 kg.
AC10 Calculated for one round per shot: 1 ton = 10 shots = 10 rounds = 100 kg per cartridge = 66,6 kg per slug/propellant = 33,3 kg per slug.
A very good match is the US 5”/54 (127mm) Mark 42 with projectile weights up to 32 kg.
Thinking about the AC10 as a burst firing gun. Calculated for a 5 slug burst: 20 kg per cartridge = 13,32 kg per slug/propellant = 6.66 kg per slug.
Best match is the 76 mm /62 Compact (US/Italy/Japan) with a slug weight up to 6.6 kg.
AC20 Calculation for one round per shot: one ton = 5 shots = 5 rounds = 200 kg per cartridge = 125 kg per slug/propellant = 62,25 kg per slug.
No actual calibers but the German 170 mm K18 from WW2 is closest (68 kg per slug).
Thinking about the AC20 as a burst firing gun. Calculated for a 5 slug burst: 40 kg per cartridge = 25 kg per slug/propellant = 12.5 kg per slug.
A matching caliber need to be below the GER/FRA 100 mm/55 Model 1964 with 13.5 kg per slug
The 50/50 rule has now reduced the calibers to smaller sizes then posted before:
AC2: ca. 45 mm (instead of 57 mm)
AC5: ca. 60 mm (instead of 76 mm - 80 mm)
AC10: ca. 76 mm (instead of 95 mm - 100 mm)
AC20: ca. 95 mm (instead of 110 mm - 120 mm)
Of course, reducing the shot size to 4 rounds per shot instead of 5 rounds per shot would restore the formerly estimated calibers. A weight reduction of 50 % per slug is just ca. 20 % reduction in caliber
Using this system on the smallest Mech artillerie, the Machine gun:
Machine gun ammunition is usually KE only. Have small caliber rounds the same 50/50 ratio found in high calibers?
Some data:
US 12.7 mm/90 M2: Propellant: 0.0153 kg. Slug: 0.0485 kg. Prop. weight = 24 %,
Case weight = 45% from total.
Rus 12.7mm DShK M38: Prop.: 0.0165 kg. Slug: 0.052 kg. Prop. weight = 24.1%,
Case weight = 47 % from total.
GER 20 mm/65 Mk 20 Rh202: Propellant: 0.042 kg. Slug: 0.134 kg. Prop. weight = 23%
US 20 mm/70 Mk2-4: Propellant: 0.0277 kg. Slug: 0.122 kg. Prop. weight = 18 %,
Case weight = 48% from total.
US 25 mm/87 Mk 38 (Bushmaster): Propellant: 0.08 kg. Slug 0.5 kg. Prop. weight = 14%
GER/Swiss 30 mm x 173 FAPDS: Prop.: 0.153 kg. Slug: 0.235 kg. Propweight = 39.4%
Case weight (assumed) = 46.4 %
RUS 30 mm/63 Ak-230: Propellant: 0.19 kg. Slug : 0.36 kg. Prop. weight = 32 %
Case weight: = 48 % from total
US 30mm Mark 46 Mod 1 (AP only): Prop.: 0.176 kg. Slug: 0.235. Prop. weight = 42%
Case weight = 44% from total.
No, instead current smaller calibers have a smaller amount of propellant compared to the weight of the slug. Propellants and slugs have much bigger deviations. For the B-tech ammo there will be a higher degree of uncertainty in the slug weight. Also from the data above. The share a cartridge has on the total weight is around 46.4% over the range from 12.7 mm to 30 mm.
5 round burst: 1 ton = 200 shots = 1000 rounds = 1kg per round = 0.536 per slug/prop.
Only the US 25mm/87 Mk38, the RUS 30mm/63 AK-230 and the US 30mm/46 Mod1 are meeting this stats.
Machine guns are determined to kill troops. Somehow I don't believe that Mechs will need a 30 mm caliber to do the job even so personal protection gear will step up too. In this case I suppose the bursts of one shot will contain more than 5 rounds. Just as an example, the weight of a heavier cal .50 round is 0.114 kg. One ton of this Ammo will contain around 8770 rounds. By 200 shots per ton a single shot would contain ca. 43 rounds.
In Mechwarrior3 like for the board game no difference is made between Clan and IS. Damage, shot size and range are the same. In Mechwarrior4, machineguns are described as MG Arrays. Innersphere Arrays have 3 MG’s (450 shots) and Clan Arrays have 4 MG’s (600 shots). That’s 150 shots for each single MG. And that’s not helpful either.
How is ammo feed organised in a Battlemech? What is keeping those rounds of a shot together? A clip, a drum with chute or something else? And what kind of containment is holding all those clips, drums etc together?
Because of them as beeing named officially ammoclips are first.
http://www.navweaps.com/Weapons/WNUS_4cm-56_mk12_pics.htm
While the rounds for ACs of the same damage class are the same the ammo feed needs to be different.
Imagine a AC with one clip of ammo. Immidiatly after the shot has cleared the barrel the empty clip becomes replaced by a new clip (“Recycle” or “Reload̶ .
An UAC needs to have two clips or at least one bigger clip fitted to the gun. For RACs there is a need to keep the ammo of at least six shots close (=ready rounds). Of course that could be done with clips too but a drum would be more practical.
And there is another aspect that is making clips unlikely. The number of shots per ton.
The UAC2 has 45 shots / toa. Under the assumption that UAC ammo is organised in clips of at least ten rounds. The last clip has to be of half size. Same thing for an UAC20.
Imagine a pilot is firing his UAC at first in AC mode. Luck is forcing him to fire his gun in UAC mode next time. That just don’t work because the clip in said UAC has only one shot left. The pilot would have to throw away the half empty clip and would need to replace it with a fresh one.
That is as silly as it sounds. Easy to understand, on RACs the whole idea of working with clips is even more absurd.
Logistics would demand paletised ammo. That would also make the “pit stop” as short as possible especially under battlefield conditions. Such a containment needs to be sturdy and it needs to be strong enough to hold the variable number of shots which can be in one ton of ammo.
Under the assumption that that ammo weights are netto the weight of said containment can be ignored.
An actual gun with B-tech stats will therefore be at least chute and drum fed. The drum will contain all the rounds of all shots in one ton as “ready rounds” like this one:
http://www.mindfully.org/Nucs/2003/GAU-8-Avenger.htm
In Battletech the size of an ammo storage inside of one Battlemech is only limited by the number of free crits. Theoretically. Therefore there needs to be a way to add extra drums with each extra ton of ammo. Imagine a “docking” chute to connect those drums or a mechanism to replace an empty drum with a new one. Such mechanisms can be made already.
On actual heavy ACs used by the naval artillery variable burst lenghts are a reality. So if the ammo is fed continously why is such an AC unable to fire on full automatic until the drum is empty?
For engaging small, lightly armored and slow moving targets a single round (high precision) or a short burst will do. Longer bursts will be directed to bigger targets or for better“saturation” of an area on long range. Very long bursts are for stopping small and very fast incoming threads like anti ship missiles or aircraft. The reasons why long bursts are reserved for life threatening situations is the tremandous wear they are causing to a gun.
A real danger, mirrored by the B-tech rules is the chance for a jam. Also on real weapons the risk for a jam increases with longer bursts.
This wear accompanied with enormous heat. Even with better barrels today machineguns are limited to short bursts and a rate of fire of 1000 rounds per minute.
Worst case, a very hot barrel can cause a freshly loaded round to cook off and destroy the gun. But on todays battlefields even a warm barrel is not wanted because of the heat signature on IR scanners.
In Battletech the whole shot thing became simplyfied for obvious reasons.
For firing longer bursts UAC will need sturdier barrels and better cooling compared with ACs. So far the heavier weight of UACs in justified in principle.
One of the things that freaks weapon nerds is the B-tech way of handling gun damage and range. Instead of just joining their rantings against Whizkids I try to find out what kind of logic might be behind those stats.
One AC2 shot can cause 2 points of damage. AC's are burst firing guns with (assumed) a 5-round burst per shot. It's all or nothing, either the whole 5-round burst is hitting the target or it is missing it completly.
Just as a hint. The US navy 57 mm/70 (2.25") Mark 110 Mod 2 can fire 220 rounds per minute. The Mark 2 loader and gun mount are 2-sided so that two types of ammunition can be handled simultaneously. Ammunition comes up from the magazine and goes into two 20-round cassettes mounted on a rail behind the gun. When filled, they move into position over the ready-use magazines and discharge their ammunition into them. There are also two intermediate 20-round cassettes (Time to reload: 120 rounds in 2 minutes). A full burst of 40 rounds would be out of the barrel in ca. 11 seconds. http://www.navweaps.com/Weapons/WNSweden_57-70_mk123.htm
Lets assume that usual bursts are reduced to 5 rounds to safe the barrel.
A 5-round burst would be out of the barrel in ca. 1.4 seconds. Such a burst is very close together and due to advanced gun stabilisation all slugs will probably hit the same target location. Unfortunatly this is not valid for very fast moving targets like aircraft and over a longer distance.
The GER/FRA 100 mm/55 Mod. 1964 autocannon (propably an AC20) has a rate of fire of 78 rounds per minute. One round each 1.3 seconds. That is 6.5 seconds for a 5-rounds burst. For hitting a supersonic aircraft this rate of fire is certainly slow. Moreso, in a fight, when both the shooter and the target are moving. The only way (to make all 5 slugs hitting the same location) is to fire from a very very short distance.
For those who haven't got it by now. The B-tech "increasing calibers have decreasing effectiv ranges" anomality is not an anomality when the most real "increasing calibers have decreasing rates of fire" rule comes into play. B-tech demands that all five rounds have to hit the same area for becoming counted. And that's doable only when the range of engagement becomes reduced with the decreasing rate of fire. 
Of course, anyone of those five, modern 100 mm artillery rounds in said 6.5 seconds burst would have devastating effects on any kind of todays targets. But there is no way to calculate the damage that is caused by, lets say, by 3 rounds that hit while 2 went missing.
I mean, 2 out of 5 missing rounds can cause a lot of (collateral) damage. Herb Beas of “classic Battletech” would call that the “aestetics of Battletech” but for me that is not a realistic approach to calculate the damage caused by a burst firing gun.
But there is another hint to answer the range question. In Battletech weapons of different ballistics are often teamed up on the same mount (Torso, Arm). Think about one medium lasers and one AC10. While the Laser is hitting straight the AC slugs will be dragged down by gravity. So even those high speed, low trajectory KE slugs can’t be fired together with the laser and be meant to hit the same area. At least not over a longer range.
Another hint. A long barrel is needed to aim a single slug or a burst on a far away target. The GER/FRA 100 mm/55 Model 1964 has a barrel length of 5.5 m. Same for the Leopard 2 120mm L55 gun. Of course, for firing only “short range” shorter barrels will do. There have been tanks with shorter barrels in history. Think about the Sherridan:
or the Sturmhaubitze 42:
or the 43M Zrinyi assault gun:
A very practicle reason for having short barrels is when the fighting happens in very confined places. Thinking about dense woodland or narrow streets.
Battletech mech models usually are sporting short barrel lenghts. The only Battlemechs with good barrel lengths for shooting long range are the Stone Rhino, Marauder, Rifleman and Jagermech. Well, wysiwyg was never a topic in B-tech.
Another point that freaks weapon nerds are the gun weights in Battletech. Before I go into this I need to describe actual guns a bit more closely. The main parts or groups of parts of a gun are the barrel with thermal sleeve screwed into the breechend, the breech with breechblock and ejector, cradle, recoil mechanism, bore evacuator, electric or hydraulic systems for aiming and stabilisation and of course the required electronics plus wiring and tubing.
Unfortunatly, even manufacturer data concerning the weight of real weapons are very confusing.
Some examples: The 120 mm L55 gun of the Leopard 2 (not an AC) has a barrel weight of around 1400kg. The whole gunsystem weights 4160 kg.
http://www.rheinmetall-defence.com/index.php?fid=1449&lang=3&pdb=1
The french GIAT F1 120mm has a recoiling mass of 1980 kg. The total weight with cradle is 2800 kg.
http://www.giat-industries.fr/asp/us/prod_otan120.asp
Both guns are competitors on the weapon market of today. A weight difference of one ton which will add significently to the weight of a tank and need to become explained. At least to the customer.
I suppose that the Rheinmetal barrel weight is realy just that. The weight of the barrel. In case of GIAT the weight includes everything from the gun that is moving when the gun is fired. That will include the weight of the barrel with thermal sleeve, breech assembly and ejector.
On the total side the GIAT data will include the barrel and breech assembly, the cradle and recoil mechanism but nothing else.
In case of the Rheinmetal gun the data are referring to the gun system. And that is including everything form above.
In comparison, the RUAG light weight 120mm gun CV90120 has a recoiling mass of 1860 kg. The barrel with thermal sleeve and bore evacuator is just 1120 kg. The elevating mass is 2600 kg. Well, parts of those extra 740 kg of “elevating mass” have to be the cradle, the recoil mechanism and gun stabilisers.
http://www.ruag.com/ruag/binary?media=87052&open=true
Another example: The Royal Ordnance L7 was once a standard in all western MBTs. The 105mm tank barrel had a weight of 1282kg. Breech and breechblock (+ejector?) are adding another 552 kg. The whole gunsystem had a weight of 2900 kg.
The competitor for the Canadian MGS Project is probably the Rheinmetal 105 mm Rh 105-20. It has a barrel weight of 800 kg and a gun system weight of 2100 kg.
How to compare those guns?
A Battletech AC20 has a weight of 14 tons without ammo. A real world AC20 the GER/FRA 100mm/55 Model 1964 has a Gun weight (tube and liner = barrel) of around 2,400 kg.
Tank guns are no Autocannons. The barrel of such a real AC is twice as heavy compared with one from those 105mm tank guns above. If this is valid for all parts of the Gun than the total gunsystem needs to weigh around 5800kg. Of course, there are a lot of uncertaintys in this calculation. But there will be a lot weight left to explain those 14 tons. “Aestetics of Battletech” again.
Unfortunatly the light ACs from tech level 3 are no way to come to relistic data either. A light AC2 has a weight of 2 tons less compared with a standard AC2. In Battletech light ACs do the same damage compared with the normal ACs. Just on shorter ranges. This is in principle a realistic approach. A real lightweight tank gun the Rheinmetall 120 mm LLR / L47 is firing the same ammo like their full weight brothers the 120 mm L55 but will do this on shorter ranges either.
http://www.rheinmetall-defence.com/index.php?fid=1461&lang=3&pdb=1
What about RACs? While there are even real examples for the biggest ACs available today that can’t be said for RACs. The biggest current Rotary AC so far is the GAU-8 "Avenger".
The GAU-8/A alone has a weight of 281 kg and is 285 cm long. The whole system (Gun, Ammofeed, and Drum) is 5,06 m long and weighs 1.830 kg (fully loaded).
By Battletech standards, this is just one heavy machinegun. The number of barrels on real RACs can differ. The early M197 20mm gun of the Cobra helicopters used in Vietnam had started with three barrels. But five to six barrels are more common (Vulcan). The GAU-8 has 7 barrels. 3 to 7 barrels will cause a certain difference in the total weight. Of course RACs will be similar “shortbarreled” like their AC cousins. Actually there is no way to predict the weight of a real RAC even for one in 45mm caliber.
Autocannons versus LBX cannons
LBX cannons are by far the most sophisticated cannons in the B-tech arsenal. Just because they are the only guns that can handle two types of ammunition. LBX cannons are firing cluster rounds as well as sub caliber slugs (APDS).
In the Battletech MW3 games the LBX is a shotgun like weapon for close range engagements compared with the AC (Ranges: LB-10X AC = 520 m, Ultra AC10 = 540 m). The LBX range became also reduced in MW4. In the B-tech novels the LBX is allways also regarded as the bulkier cousin that comes into play when actions are becoming personal and nasty. Like the "roomsweeper" that those wild west sheriffs have used to call a saloon brawle to order.
But that's not true according to the original board game stats. LBX are not shotgun like. Instead they are firing for longer ranges than their full bore slug firing cousins.
AC2: 6 tons, 1 crit, damage 2 points, 45 shots per ton of ammo, Range: 8, 16, 24
LBX2: 6 tons, 4 crits, damage 1 per pellet, 45 shots per ton of ammo, Range: 9, 18, 27
AC5: 8 tons, 4 crits, damage 5 points, 20 shots per ton of ammo. Range: 6, 12, 18
LBX5: 8 tons 5 crits, damage 1 per pellet, 20 shots per ton of ammo. Range: 7, 14, 21
AC10: 12 tons, 7 crits (2 crits in MW4, damge 10 points, 10 shots per ton of ammo. Range 5, 10, 15
LBX10: 11 tons, 6 crits, damage 1 per pellet, 10 shots per ton of ammo. Range: 6, 12, 18
AC20: 14 tons, 10 crits, damage 20 points, 5 shots per ton of ammo. Range 3, 6, 9
LBX20: 14 tons, 11 crits, damage 1 per pellet, 5 shots per ton of ammo. Range 4, 8, 12
The ability of the LBX to fire subcaliber slugs is generally ignored in the novels (as it is in the computergames). In MW3 the board game stats are taken for AC10 and LBX 10. In the MW4 the weight became raised (AC10 = 13 tons, LBX = 12 tons). In case of Clan weapons the weigth stats of LBX is idendetical with UAC guns of the same damage class.
The ammo that makes the LBX unique is the cluster round. Unlike a canister round (grape shot, buck shot, chain shot, etc) a cluster round is leaving the barrel as one containment which opens up short before the impact and peppers the target with sub ammunition (pellets). In difference to canister rounds a cluster round needs to have a programable fuse like modern AHEAD ammo.
Like for the AC2, the LBX2 gun ammo comes with 45 shots per ton. Each of the pellets fired by a LBX gun can do a damage of one point. Just imagine the LBX gun would fire one single round per shot. For comparable damage (to ACs) one LBX2 round needs to contain two pellets. A LBX5 round will therefore contain 5 pellets, a LBX10 round will contain 10 pellets and a LBX20 round will contain 20 pellets. Of course that is stupid. Today’s cluster rounds of smaller caliber guns are containing more pellets (or more likely "flechets") per round and they are firing bursts.
So if the number of shots per ton is alike may this be also valid for the number of rounds per shot? Are LBX, like ACs, burst firing weapons?
No, because the damage is calculated per pellet and there are only 2 pellets in one LBX2 shot. If the pellet is the basic unit that can cause one point of damage than the calculation does not add up for a burst. THE LBX HAS TO BE A SINGLE ROUND PER SHOT WEAPON.
The makers of MW3 have truely screwed up here.
LBX cannons are also firing sub caliber slugs. The amount of damage of one LBX cluster round (100 % hit rate for all pellets assumed) has to cause the same damage like its one-slug (APDS) brother and like one 5-round APDS burst of it's AC cousin. In case of the LBX5, one single pellet causes the same damage like one slug out of the five fired by the AC5. Taking the 50/50 calculation for propellant and projectile is the only way give said projectile the necessary kinetic energy. See above for the mode of calculation:
LBX2: 7,45 kg per cluster round .
A matching caliber needs to be above the 76 mm /62 Compact (US/Italy/Japan) with a slug weight up to 6.6 kg and below the the UK 3.7 inch Howitzer (9.1 kg slug in 94 mm caliber).
LBX5: 16,5 kg per cluster round.
Best match is the Russian 100 mm/70 (AK-100) with a warhead of 15.6 kg. Next best would be the GER/FRA 100 mm/55 Model 1964 with 13.5 kg per slug
LBX10: 33,3 kg per cluster round.
A very good match is the US 5”/54 (127mm) Mark 42 with a projectil weights up to 32 kg.
LBX20: 62,25 kg per slug.
No actual calibers but the German 170 mm K18 from WW2 is closest (68 kg per slug).
Compared with the AC calibers from above:
LBX2 caliber > AC2 caliber ca. Factor 2.
LBX5 caliber > AC5 caliber ca. Factor 1,8
LBX10 caliber > AC10 caliber ca. Factor 1,6
LBX20 caliber > AC20 caliber ca. Factor 1,8
Surprising, isn't it? No wonder that LBX are described as bulkier as ACs. They need to have nearly two times the caliber of its AC cousins.
Anyway, the calculations for slug and propellant of a B-tech cannons are assumptions based on actual ammo. I don’t belief that the devellopment of a more potent propellent (and stronger barrels to withstand wear and gas pressure) will lead to a reduction of the propellant load. Remember, speed is more important then mass. And Newton’s law is still valid also in the Battletech universe.
Because of the LBX gun being a one round per shot weapon many problems usually related to bursts don't exist. Burst weapons have the problem of keeping the aim on the target during the burst as described above. This problem isn't limited to artillery. With the caliber also the mass and recoil of a gun will increase. Everybody who has ever shot an large caliber assault rifle and a submachine gun (in full auto) in comparision knows what I'm talking about. Actually the PzH 2000 / MONARC project is aimed to fire 155 mm artillery (44.5 kg per slug) at "full auto". By now this is realy the technological edge. With such a caliber the rate of fire is decreasing to 10 to 12 rounds per minute. That's 6 to 5 seconds per round and 30 or 25 seconds for a 5-rounds burst. 25 to 30 seconds cannot be regarded as a burst in my understanding. Therefore an LBX cannon with an assumed caliber between 155 and 170 mm as a single round per shot weapon is fitting much better also into the current situation.
All cannons on a Mech need to have autoloaders. But due to their behavior as single round per shot weapon the B-tech LBX are closer to actual tank guns than to naval artillery. Unless nobody is inventing Ultra LBX. The wear on a barrel of an LBX will be smaller. That allows for a less sturdy design. On the other hand, the safed weight will be eaten up due to the higher gas pressure related to firing bigger rounds.
In difference to those 5 AC slugs, all 20 pellets from an LBX 20 round are arriving on the target at the same time. Simply said, on a one round per shot weapon the negative effect of a slow rate of fire cannot be applied. Of couse, some pellets will miss the target. But that will allways be the same percentage depending on the speed and size of the target. Remember, the cluster releases the pellets depending on its distance to the target and not depending on its distance from the gun barrel. This explains also why cluster rounds are having an effectiv range greater than those of autocannons. In principle, also in Battletech, bigger calibers have an increased effectiv range.
On the other hand there are no other reasons than game balancing for sticking to the B-tech ranges. As stated above, single round per shot guns can have much higher ranges at least up to the limits of current battlefield conditions (3000m at day/ 800 m at night).
Gun and ammo devellopment.
During the 60th The tank killing effect of hollow and shaped charge warheads (develloped during World War II) was so owerwhelming that all tanks had this kind of Ammo (HEAT = High Explosive Anti Tank). During the 80th and 90th the armor of MBT's became enhanced in a way that only APFSDS KE ammo was able to break it.
Anti tank missiles (which can't get the necessary speed) became equiped with dual shaped warheads to stay effectiv. For the Abrams main gun, the US army actually does R&D nearly exclusivly on KE Warheads.
http://www.geocities.com/futuretanks/armorsachillesheel.htm
During World War II the most effectiv anti tank caliber was 88 mm. Until the end of the last milenium the gun calibers have risen slowly. In Nato country’s from 90 mm (early M60) over 105 mm (Later M60, AMX 30, and Leopard 1) to the 120 mm of actual Abrams and Leopard 2 MBT’s. In the warsaw pact until its collaps a similar devellopment went from 100 mm (T55) over 110 mm (T72) and 115 mm (T80) to the most recent 125 mm (T90). Interestingly, all those elder calibers are still in service. Even the 90 mm caliber ammo became enhanced and is like the 105 mm ammo still used on so called medium and light tanks.
For naval guns the high caliber autocannons of today are following a different path. The huge battleship guns of World War II are all retired to museums or scrapyards by now.
As far as I know, the US Battleship gun barrels have started a new but comparably short lifed career as fuselage for the new Bunker Buster Bombs.
With the upcoming of guided missiles naval guns got a different role as jack of all the trades a missile can’t economicaly fullfill. Naval guns in Nato country’s are using a small range of high calibers ( 57 mm, 76 mm, 100 mm, 127 mm). The british 114 mm caliber is a bit extraordinary. Former Warsaw pact countrys are using 45 mm, 57 mm, 76.2 mm 100 mm and 130 mm guns. The russian 100mm and 130mm guns have water cooled barrels and will need an extra heatsink . Current research is aimed to bring guided 155 mm ammunition from land warfare onto warships.
While the tank gun calibers where going up the calibers of the APFSDS penetrators became reduced. Laws of physics again. As more needle like a penetrator becomes as more armor it can penetrate effectivly. To keep up the weight in those thin rods the lenght need to be increased too. Actuall penetrators have a caliber to lenght ratio of 1:22. Those rods need to be fin stabilised. A twist stabilisation would not be enough to prevent tumbling. Rumors say that the russian K5 activ armor is able to defeat these penetrators too. If that is true than gun calibers will go up another step.
But there is an actual weight limit in western tank ammo. In NATO tanks ammo is usually loaded manually by a loader. And the total weight is growing faster than the caliber.
90 mm (M690A2): Round weight 11.5 kg.
105 mm (M900): Round weight (DU): 18,5 kg.
120 mm (DM 43 A1): Round weight (Tungsten): 20 kg.
Handled manually ammo weights above 20 kg will certainly put a dent into performing “rapid fire”. Unless western armys are not adopting autoloaders the actual 120 mm gun calibers will be the edge. Rumors are, that Rheinmetal is already testing a 140 mm gun on a Leopard 2. I suppose those rounds will be loaded with an autoloader.
Another limiting factor is the ammo storage capacity of a Tank.
The Patton M48A3 was able to carry 60 rounds for its 90 mm M41 main gun. The Patton M48A with an 105 mm M68 gun was able to carry 54 rounds. Rumors say that an Abrams M1 was able to carry 55 rounds (105 mm) but the Abrams M2 is limited to 40 rounds (120 mm). The Russians needed a lot of technological trickery to get 45 rounds of 125 mm caliber in one T80 tank.
Tanks unlike airplane aren’t able to detach easily from the enemy for a reload. The small storage capacity for 152 mm rounds inside of the M551 Sherridan were also a reason why this system didn’t last long in the US arsenal. So far, B-tech got that right and a tank with just 5 or 8 rounds for the main gun has certainly a problem on any modern battlefield.
In this regards Battlemechs have less trouble. Modern chute technology allows to transport ammo from everywhere in a mech. May be even from the legs. And while a tank has usually only one turret on top of its hull a Battlemech has at least two "turrets" attached to its shoulders and may be a third one on its back (StoneRhino).
In principle an Autocannon can fire on a target until the target is destroyed, its ammo is cooking off or it is out of ammo. The number of rounds used to destroy a target depends on the armor grade of the target. The same gun that just needs a single short burst to ruin the day for infantry can be used to hunt helicopters or aircraft and is able to kill an armored vehicle with a longer burst too. Real autocannons are also able to handle two different sorts of ammo.
http://www.rheinmetall-defence.com/index.php?fid=2420&lang=3&pdb=1
Also to consider, in comparison the hunting of infantry with 120 mm KE rounds is a waste of energy. For this reasons smaller caliber autocannons are more versatile compared with their LBX cousins (tank guns). I wouldn’t be overly surprised to see an Leopard MBT with an 105 mm AC gun. May be not in the Leopard 3 but may be in the Leopard 4. A gun like that is actually in devellopment for the Canadian MGS Project.
http://www.sfu.ca/casr/bg-mgs-project.htm
Hmm, 18 rounds per minute. That’s one round in 3.33 seconds. A 5 round burst would be out of the barrel in around 17 seconds. Not bad, but the GER/FRA 100 mm/55 Mod. 1964 autocannon was already firing at a rate of 78 rounds per minute. So far, high caliber ACs on tanks have a way to go.
Of course everything in this regard will be depending on future developments in armor technology.
Vehicle Autocannons have evolved especially as AA weapons and the calibers have been slowly risen from 20mm over 25mm and 30mm to 35mm. The russion 57 mm AA was one of the most successfull AA guns during the vietnam era. In the real world the development of high caliber ACs for land warfare became stalled and missiles have taken over. The biggest development to overcome this stall was the invention of a fuze that can be programmed when the shell is leaving the gun mouth (AHEAD ammo = LBX Ultra AC cluster round). The importance to defend structures of high strategic value like Nuclear power plants against incoming air threads is growing and therefore higher caliber AA guns have a chance to come back.
So far in naval artillery any caliber made auto as fast as the necessary technology became available. UAC 20’s are a reality in naval artillery of today and they will become a reality in tanks too.
Twist stabilised APDS and F-APDS will make their way into space combat as fast as this becomes reality. That will also lead to the come back of twist stabilised subcaliber sabots without fins like the old penetrator design of the russian BM-8.
A current development not covered by this study are guided gun projectils. Actually those are limited to heavy artillery and mortars. For current high speed KE ammo the guiding systems aren’t fast enough and on short ranges there isn’t much sense in using them. Neither are barrel fired AT missiles.
So far, for ACs, UACs and LBX guns (except LBX20) used in Battletech it is possible to find matching counterparts today. At least for the ammunition. All of them are sporting a single gun barrel. The only sort of guns which hasn’t even an AC2 equivalent in the real world are the RACs.
There is a way to explain the weight stats for Battletech weapons in general.
Up to Mechwarrior 4 the weaponbag system allows to put any kind of equipment in any part of one mech as long as it is not bigger than the awailable number of crits. Wether a weapon has recoil or not doesn’t matter. On Omnimechs this ability is even enhanced. Today there are two ways to get this done.
The first way would be by using weapon pods. Weapon pods are used today especially on aircraft and helicopters. They can either have an external ammofeed:
Or have the ammo stored inside with the gun:
This second method would lead to One-Shot ACs because those pods can’t be reloaded easily.
The other way would be by putting those guns in at least partly convertible weapon bays.
Unfortunatly Battletech models sporting both. So which might be the Battletech way? The credo of Battletech is keep it simple. And for keeping it simple all weapons have the same weight independently from the way they are mounted. Wether they are handheld like the medium laser on a Stinger, the PPC of a Battlemaster, or the AC5 of a Wolverin (WVR-6R) doesn’t matter. Wether they are slung to an arm or essentially are the arm doesn’t matter either.
No matter how, their presence or absence does not effect the maximum armor points or structure points of the mech. And that is possible only when weapons are essentially not armored (Would be very silly, right?) or when they can have their own armor (weaponpods).
My hypothesis is that for keeping it simple, the weapon weight data are maximum weight data for a handheld, individually armored version of the gun in question. When the weapon is build inside an armored housing of an arm or a torso than it will be without armor. Otherwise that would alter the armor stats of that area. That is may be a stupid hypothesis but it would help to explain in general the overweight of Battletech weapons compared with real weapons.
The ranges of Battletech guns are smaller than those of their real counterparts but this is made for ballancing the board game. In reality the ranges of engagements haven’t changed much since World War II and only a little during the last 50 years. All those enhancements in the development of cannons and ammunition have been focused on accuracy and armor penetration. The ranges on a battlefield are usually dictated by the nature of the battlefield. But even on an open plain the hitting of a moving vehicle is a challenge on ranges > 3000 m in daylight. In the night the maximum range decreases to 800 m. For Battletech story’s as well as for RTS I would recomment not to go below these ranges.
Jager A mech in its bay is safe. But that's not what mechs are made for.
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