I actually think this has a lot to do with this:
Don't forget the ranges that these same weapons get when tacked onto an Aerospace craft, and the flight times which suggest "Ludicrous Speed!!!" as the velocity at which they travel.
Projectiles in BT, in order to get the kilometers space ranges in the 10 or less (most obviously less) seconds one can expect it to fire and still hit the target on time, are moving at insane speeds that modern ballistic engineers only dream about.
BT Missiles aren't armed while in the tubes. They arm themselves in flight. I imagine that inside the first 6 hexes, they've only been in the air the smallest fraction of a second.
How does this apply to ACs? Have you notice how they group their damage? It's all or nothing with an AC. At least with Missiles, you get partial cluster sizes. With ACs, it's full damage for a group, or it's a miss.
While I'm one to laud the accuracies inherent in a futuristic weapon tied to a futuristic targeting system, ACs do have the high recoil to deal with. The larger class ACs have massive recoil to handle for the shear power they're pumping out inside a second or two. It's all they can do to keep the AC on a large target, let alone expect it to have all its shells slam home on a specific area on that target.
I'm one that follows the idea that if you don't group the shells or the shot well enough, the armor, or the defensive stance of the Mech combined with the armor, deflects the shots 'harmlessly'. If you spread your shots across multiple locations, you just got a miss. In game terms you don't get to roll for location and color in armor or internal structure boxes.
If the AC, and the Mech itself, are fighting the recoil of the weapon, then it seems like a true hit is next to impossible. But, it happens with some regularity. Why? Because I think there's more to the ammunition ACs use than is told to us.
I posit that BT ballistic munitions are smart rounds, that home in on a designated target via controlled spin or fin stabilization, or whatever. If a weapon groups its shots well enough, they'll all home in and do the damage, otherwise, they spread out too far and do nothing more than 'scratch and burn the paint'.
The lighter Autocannon classes are dealing with the same recoil. Sure, they're not trying for massive groupings, but they are trying for range, and to get that range, the rounds have to pour out the barrel at a greater velocity. Some limitations could also be attributed to the size of the rounds, as well. But, that extra fraction of a second is all that is needed to keep the spread from homing in successfully.
The longer I play, the more I try to mentally visualize what's actually happening, the more I come to believe it's not any one thing that allows for a certain aspect to happen. It usually ends up being a combination of factors which may or may not overlap with other game aspects.
Well to me LBXs firing Cluster seem to be using large scale AHEAD rounds, basically advanced proximity fuzed rounds, smiler to the Airburst ammo used by the XM-25 (speaking of which IIRC the AFFS also has this for rifle & shotgun ammo in the RPG).
Assuming B-tech autocannons are flinging rounds at very high speeds, one way to improve velocity is to deal with a light weight shell fitted with a shoe to fit to the barrel, Sabot rounds (I.e. APDSFS rounds) which dose not need to be only used on kinetic rounds HEAT rounds also use it (I.e. M830A1). Though Autocannon projectiles are an odd item, their fluffed as being HEAP, but they also are often mentioned as having DU tips, the closest rounds I can find that would seem to be what B-tech uses is the Mk 211
Raufoss (listed as
HEIAP -High Explosive Incendiary Armor Piercing).
One technically dose not even need to go into capital scale to see evidence of high speed projectiles.
AA shooting can show this.
B-tech ASFs can be traveling at a velocity of 12, or 2,160kph (mach ~1.8), now I never heard of a good anti aircraft gun that fired shells that traveled at velocity's slower than the intended targets, and most successful guns seem to fire shells that are easily three to six times faster than their intended targets (I.e. the Flak 18 fired rounds at ~800m/s it's targets often flew under 200m/s), this is still largely true today, most real world AA guns still fire rounds in the 1,000m/s range but the "targets" still often fly under 300m/s. As B-tech Autocannons are considered some what effective in this role, it would seem to be that the rounds they fire are noticeably faster than their targets.