I have geeked out a modular AC system that lets you choose any damage with any range and get 3025 appropriate tonnage before.
The AC12 and AC/15 issue is that while you can make any specific combination work, I think what you are trying to do here is fill in the ac/10 to ac/20 gap. The ac/10 has 150 damagexrange, while the ac20 has 180 damagexrange. I would set the damagexrange at ac/10 levels, to keep the ac/20 more relevant. The end result is an ac/15 with 10 hex range (the ultra20 range) and 6 shots/ton and an ac12 with a 12 hex range (the desired 4/8/12) and 8 shots/ton.
Both ac/12 and 15 would weigh 13 tons as the midpoint tonnage, though the suggestion for 12.5 tons for the ac12 by cannonshop works too, and I can see that both guns would be unpopular lore wise as they wouldnt have a long enough range to qualify for a LOS range band, making them more like variants of the ac20--the ac/10 with its ability to shoot several kilometers would still set it apart from all the 'headcapper' cannons which are more like bazookas/recoilless rifles if we look at their game stats.
You're basing on average damages, right?
Here's what I got:
AC/10
!0 shots/ton (10x10=100)
100 potential damage.
AC/20
5 Shots/Ton (20x5=100)
100 potential damage/ton of ammunition.
AC/15
x shots/ton does not equal 100 damage, it either goes under, or over. (6 shots/ton=90 potential damage, 7 shots/ton=105 potential damage) There's no 'clean' way to divide 100 by 15.
AC/12
Same problem, you're either going under, or over, but no clean way to divide 100 by 12 and get a round figure.
which is probably why we don't have those in the books. The only outlier among common 'baseline' autocannons is the AC/2, which COULD be 50 shots, but is 45 instead, for a total of 90 potential damage, which would be the same as an AC/15 with a six round/ton magazine.
The range bracketing is easier, because there's only one intermediate range bracket between an AC/10 and an AC/20, and it's a nice round figure at 4/8/12 (one hex longer range than an AC/20 and one hex shorter than an AC/10, which fits the pattern from the 5 to the 10, but not from the 2 to the 5.)
The most logically consistent one to enter into fitting the pattern, is the 15 with a six rounds/ton magazine, as it computes to nice round numbers that are easy to retain, BUT...
it bumps up against the Large Laser for ranges, while sharing the damage of the Gauss rifle.
the 12 still gives a potential headcapper with more range than a 20 but lacks the knockdown. at (12x8=96) it's got a non-divisible by ten and still below 100 damage potential per ton of ammunition (absolute maximum value per ton of ammo)
I didn't address heat in my entries, because I...didn't. It would certainly have it, and produce it for installations on a 'mech, (these aren't gauss rifles after all).
but most of it slots, and is easy to track without complexities, which imho is a critical feature of any fan weapon added to the game, and they can scale with the tech pretty easily (the 15 more than the 12, I'm afraid), with LBX versions reaching the same base range as a baseline AC/10 while providing that headcapper experience (ditto for Ultra versions).
I'm not 100% on what their average potentials would calculate out to, only what their absolute static-target or internal ammo explosion damage potentials would be per ammunition ton.
