-What's an Autocannon?
-Official FASA description
-Self loading or automatic weapon?
-If automatic weapon, then what caliber? What ROF?
-how many rounds are in an AC shot?
-Why an automatic weapon?
-why not single-shot?
-why are Acs so large and heavy?
-why they have such lousy ranges?
-why bigger cannons have shorter range?
-Rifled guns or smoothbore? (Ultras and LB-Xs)
-What ammunition they use?

More…
Thrown weight of shot and damage
Overheating
Why not DU rounds instead of tungsten penetrators ?

RECOIL
 
 
 

   1)      What's an AC?

The official FASA description from Battletech 1st Edition is that an AC is
“an automatic quick fire cannon”

So, it's an AC a self-loading ( auto reload ) like those in QuickFire guns
mounted in ships today or like the T-72 tank gun, or it's an automatic
weapon like a machine gun?

   2)     If an AC indeed was just a self-loading gun, the term “auto” is
       redundant, as it's obvious that in ‘Mechs at least, the gun is
       automated. Moreover, all the evidence from the novels and the Tech
       Readouts says explicitly that an autocannon fires bursts of
       projectiles, like a large scale machine gun.

If it's indeed an automatic weapon, then what caliber and what Rate of Fire
(ROF) does it have? Can you imagine a tank gun firing dozens of bullets like
a machine gun? Me neither.

   3)     Caliber and ROF

FASA fluff gives us a good idea of “where the shots are headed”, as the
saying goes, on both ROF and caliber, and from the scale of the ‘Mech
drawings, we get an approximate idea of what size ACs are.

To answer these questions we have to consider how many individual rounds are
in an AC “shot”. It's obvious by now that an AC “shot” is not a single
projectile, as much as some like the idea of an AC/20 firing a 200 kilos
shell with each pull of the trigger. And moreover, a vital consideration is
that the ammo tonnage represents both the projectile fired and the
propelling charge.
 

Therefore, I took ammunition sizes and weight tables, and compared the ammo
loads per ton of each AC type, to confirm the educated guesses I made about
their caliber.

Here are the results:

AC/3
Caliber: 30mm [ source TRO 3026: SarLon AC/2 "Firing a ten-round burst of 30mm hypervelocity slugs with each pull of the trigger" (p.16) ]
Volleys per ton: 45
Rounds per volley: 10
Total rounds per ton: 450
Weight of each round (projectile + cartridge): 2.2 kg
ROF (assuming a 2 second burst): 5 rounds/sec = 300 rpm
Effective Range: 2400 meters ( assuming 1 hex = 100 meters )
Special Cases:
- Ultra AC/3 burts: 10 rounds/sec = 600 rpm ( high, specially considering the size of the cartridge, and resulting in serious overheating, but feasible )

Present day equivalent: the 40mm Bofors gun used in the US Army M42A1 Duster AAA tank gives an approximate idea of the round size, but the AC/3 round is actually *twice* as large. A 40mm Bofors rounds weighs about 1kg, while the AC/3 round fires a smaller projectile and has a cartridge that is two times as long. Picture a round that its cartridge case is more than 50 cm long, with a small 30mm shell on top. Now you can understand why this damn thing can still have the ability to pierce armor at 3,000 meters, and still have such puny effect even at close range. All it does is nice little holes. And it would be very likely that the small, lightweight, slender projectile would shatter on impact from the stress if fired at close range, instead of drilling through the target and exiting on the far side as might be expected. Amusingly enough, it looks like there was a reason for that absurd minimum range restriction,  though that was an attempt to legislate tactics through game mechanics, and this is a ballistic quirk. Pure coincidence, though.
 
 

Commentary: Why the AC/3 is a 30mm and not a 40mm like it's closest match
the Bofors gun?
To begin with there are substantial differences between an antiarmor gun and
projectile, and their respective antiair counterparts. A flak shell needs to be of a certain size to have enough explosive to make a big bang in the air, so even if the targetted aircraft is not directly hit, it still gets rocked by the explosion, or it's hit by fragments.In contrast, small caliber autocannons like those of today (case in point,the 25mm Bushmaster gun in the Bradley) follow the pattern of small arms ammunition of large cartridges and small projectiles, with the goal of achieving a very large propellant charge size to projectile mass ratio to get the highest muzzle velocity possible, and therefore, a high armor penetration ability. Furthermore, the 40mm Bofors has a muzzle velocity of only 875 m/s, and if the AC/3 was to have any effect at all against armor, it would need a muzzle velocity of around 2000 m/s, considering the size of the cartridge, that certainly would be the case.
 

The AC/3 combines features of a machine cannon and a full size tank cannon. It has the former's authomatic mechanism and high rate of fire, wich is impresive considering the round size, though is to big to fire long burts or keep a sustained rate of fire without excessive overheating or barrel wear. and it has the muzzle velocity of an armor piercing round, and it resembles a mid WWII antitank gun, the Panzer III L/60 50mm would be an approximate equivalente.
I agree that a larger shell is more effective when firing HE, and that with sabot rounds we get rid of the restrictions of the slug to cartridge size ratio. But then, if you increase the diameter of the projectile, you also have to increase the diameter of the cartridge, and that increases the size of the breech block (remember, volume of a cilinder is Pi*Radius Squared*Height, a wider round needs a more massive breechblock than a slender, larger one)  and the bigger shell means a larger and heavier barrel, a weight increase that’s compounded by the need to armor the barrel.
Of course a 40mm AC would be superior, but you’d have to pay for it with a
weight increase. If damage increases, and range remains constant, then the
mass of the weapon increases, and the ammo load is reduced.

At this scale, the 30mm caliber is the optimal choice, for the roles a gun
this size and ROF could conceivably be used for:

1)     A long range "sniper" gun with a high muzzle velocity and flat trajectory that using APDS rounds can still damage armor at long distance, while at the same time has a controllable recoil that ensures it's accurate enough to get a close grouping of hits, neccessary to make a dent on ablative armor. The drawback is that the small size and mass of the subcaliber penetrator means that each individual round effect is weak, and the overall effect is more like pinpricks. In other words, though penetration power at close ranges would result at best in punching neat little holes, in the best of cases, while a thick enough level of armor protection would result in the penetrator rebounding or fragmenting after    penetrating some [ Note: I’m not pulling this out of thin air , it's the   actual ballistic behaviour of small mass penetrators. This was what happened with the British 6 pounder (57mm) AT gun during WWII, despite the use of APDS rounds, the slug was too light to drill through armor all
the way to it's theoretical armor penetration ability ]

2)     Those same ballistic properties make it suitable for the dual purpose as an AAA gun, employing proximity fused HE shells.

3)     While APDS rounds would be used to attack armored targets and for long range firing, the round design allow’s for the optimal performance firing solid API rounds, for use against soft targets and specially, vehicles.

4) Finally, the high ROF makes it useful for antipersonnel suppression fire
using HE shells,  there’s no great difference in the area of effect of a
40mm shell versus a 30mm one, and the 30mm has the advantage of a higher ROF
that means a greater area of effect, and that the ammo load per ton is
larger.

Larger calibers are ballistically possible, but the modest increase in
performance is probably not worth the effort.

As the game is written, the game stats condition ballistics, there’s little
room to maneuver around it, further confirming the rule of thumb that every
10mm of caliber is 1 damage point, a figure convenient for our analysis.

Side note: If you insist, a Class 4 AC firing 40mm rounds would have the
following stats Heat 1 Damage 4 Short 1-7 Medium 8-14 Long 15-21 Extreme
22-28 Tons 7 Criticals 2 Ammo 30

AC/7
Caliber: 75 mm [source , Divine Revelation :)]
Volleys per ton: 20
Rounds per volley: 5
Total rounds per ton: 100
Weight of each round (projectile + cartridge): 10 kg
ROF (assuming a 2 second burst): 2.5 rounds/sec = 150 rpm
Effective Range: 1800 meters ( assuming 1 hex = 100 meters )

Present day equivalent: the Ares XM274 75mm autocannon of the late 70s-early 80s experimental tank HSTV-L of AAI Corp. Better known as the LAV-75 in the TWILIGHT 2000 RPG

Commentary: Once I figured out that ACs fired bursts of projectiles, I wondered where did the idea come from. Lacking evidence, I assumed the writer had concluded that the next logical step after the quick fire naval guns, or tank cannons equipped with autoloaders, would be a fully automatic tank gun. In 1985, that was sci-fi, or so I thought. Later, I found that in Twilight 2000 there's a tank, the LAV-75, that's equipped with a true autocannon. I figured out this was a 90s military project, that was unknown to the public at the time of the writing of Battletech (1985), and that was picked up by GDW as an example of future weapons that would be used in World War III.  Having something solid to work on, aside from sci-fi speculation, I modelled the AC/7 after this weapon, and extrapolated the characteristics of the other autocannons from this starting point. It was not until 1999, when I started collecting the Osprey Armor series book, that I found that autocannons are real and not a project, and that it even has an official US Army designation. The information and a picture of the prototype tank can be found in the last pages of the book US Light Tanks 1944-1984 by Osprey Publishing. The ammo weight is listed in the Twilight 2000 charts. The Ares 75mm autocannon was one of those 80s military research projects wich was used to base Battletech weapons on, by means of an unknown author working for Hero Games,  and the technical background was borrowed in turn by FASA.

WARNING! HEAVY MATH AND BALLISTICS FOLLOW !!

The Ares gun has a slightly higher ROF of 180 rpm, an effective range of 1600 meters ( vs 1800m in the AC/7 ), because at longer ranges it has lost so much energy that it lacks penetration power.

Trouble is, that the performance of a 75mm 10 kg cartridge using modern APFSDS slugs is largely guess work, as the only rounds of that caliber are WWII ammo and cannot be compared with a modern round performance.

The original writer didn't pull the damage values out of thin air, he must have had a compared ballistics chart where he drew the damage scale.

My intent was to reverse the process to try to find the matching gun calibers with the AC classes.

What struck me as odd was the poor performance of the 75mm Ares gun. It  has less than 40% of the penetration value of the 105mm gun standard for NATO armies for 20 years. Though this would support the Battletech original statistics, I knew that the damage values for the AC/2 and AC/5 numbers were chosen more because they were nice looking and fitted in a damage scale that increased twofold per step, rather than actual ballistic performance.

If I worked downwards from the 105mm, the theoretical penetration for a 10 kg 75mm shell should be 50% of the 105mm round at least. This supports the notion that an AC/5 should do 5 points of damage.

But then, in another sourcebook I found out that for the WWII 85mm gun of the T-34/85 tank, its shell was listed as weighing 10 kg, and firing a High Velocity Armor Piercing round, a primitive version of the sabot round, has the same penetration ability as the Ares modern gun firing much better ammunition !

This confirmed what I suspected from the gun's overall appearance, stats and the LAV-75 chassis, that it's a low pressure gun, with a lower muzzle velocity ( and hence, less penetration ability ) than what might be expected in order to reduce recoil. A 15 ton vehicle firing a 75mm gun would receive a good kickback from it, let alone one that fires a three round burst. It's very likely that the LAV-75 prototype had almost as much trouble with the main gun recoil as did the tank it was supposed to replace, the 16 tonner M551 Sheridan.

Back to Battletech, the 75mm round of an AC/7 should have 60% of the power of the 105mm round that is the basis for the AC/10. So the original AC/5 should do 6 points of damage, that fits nicely with the revised damage value of the AC/3, so it may seem the AC/7 is slightly overpowered, but I stick to the 7 point figure because a) it matched the gun caliber and was conveninet for naming and reference purposes b) it was consistent with the scale that emerged, that each 10mm in caliber equal 1 damage point and c) most important of all, the extra damage pip represents the cumulative effect of the slightly higher rate of fire of the AC/7 compared to the AC/10. The AC/7 fires 5-round bursts and the AC/10 4-round ones.

AC/10
Caliber: 105 mm ( source , Divine Revelation :)
Volleys per ton: 10
Rounds per volley: 4
Total rounds per ton: 40
Weight of each round (projectile + cartridge): 25 kg
ROF (assuming a 2 second burst): 2 rounds/sec = 120 rpm
Effective Range: 1500 meters ( assuming 1 hex = 100 meters )

Present day equivalent: the 105mm round of the L7A1 tank gun standard of Western armies from the 60s to the early 80s is the exact equivalent.

Commentary: The present day 105mm round weighs exactly 25 kg, confirming  the accuracy of assumptions regarding caliber and rate of fire. Considering the time of the writing, it's logical to assume the author patterned the standard autocannon, after present day tank guns, and considering the author was American, and that at the time ( circa 1984 ) the US Army hadn't yet upgraded to the German 120mm Rheinmetall smoothbore gun ( a different story altogether ) it's certain that the AC/10 was patterned after the 105mm, wich had been a NATO standard for 20 years.

So far so good, round, ROF and damage potential look right, but when we examine range, Battletech rules and reality begin to diverge. There are a host of reasons that explain the discrepancy that are dealth with in the following sections. Here I'll comment only on the ballistic properties.

To make a long story short, what's relevant is not the actual range of the round, but the effective range. It's irrelevant that a 105mm projectile can reach up to 4,000 meters, if the majority of engagements are fought at ranges below 2,000, other than in the desert, visibility and direct Line Of Sight is restricted in most terrains far below that. Add to that the technological decay and the ensuing loss of precision, and a range of 1,500 meters for the AC/10 isn't bad at all. ( Note: If you still object to this, I remind you the Soviet Army tank gunnery doctrine was based on this reasoning. Don't bother in ensuring accuracy at ranges that belong more into the theory than in most real-life experience. Of course, this thinking backfired on Soviet equipment in the desert against Israel and the US, but that doesn't mean the doctrine is wrong, but inadequate to a set of circunstances and adequate for others )

To summarize, adding up visibility, tactical, technological and other considerations, it's not a good idea to fire at ranges beyond 2,000m even if you could:  because accuracy drops drastically making such extreme range shots a waste of ammo, and ammunition is *heavy* and autocannons spend it very fast. Is better to save your shots for when you have a good chance of hitting.

The other point I have to make is that is not enough to hit the enemy, you must have the ability to cause damage. Due to the conditions imposed by the game technological background, Battletech stresses that precision "one shot-one kill" weaponry has been replaced by a focus on volume of fire "if you throw enough stuff at the target, not only will you hit, but some of it will cause damage". This is particulary true in regard to ballistic and missile weapons.

Aside from the targetting limitation, discussed above, there's an ammunition constraint. From the Max Tech expanded chart, the extreme range for an AC/10 is 20 hexes, or 2,000m ( remember 1 hex should be 100m for range purposes ).
At ranges of 2000 to 4000m, you could still hit a target, but at those ranges, the penetration of the standard APFSDS round has dropped by 50%. It's not practical to waste your limited supply of ammo in difficult shots that even if you achieve a hit, will cause a reduced effect.

The range of an AC/10 is the effective range where the highest accuracy, and highest damage potential combine.
 

AC/20
Caliber: 150-200 mm ( various sources: TRO 3026 Scarborough Original: 200mm, (p.32); Crusher SH Cannon: 150mm (p.42); Zeus 75 Mk IX "fires a four-round burst" (p.46); ChemJet Guns: 185mm (pp.68-70)
Volleys per ton: 5
Rounds per volley: 4
Total rounds per ton: 20
Weight of each round (projectile + cartridge): 50 kg
ROF (assuming a 2 second burst): 2 rounds/sec = 120 rpm
Effective Range: 900 meters ( assuming 1 hex = 100 meters )

Present day equivalent: