they just really aren't that great to be honest. Oh I use them now and then, but they are freaking heavy, have very limited ammo, ammo blows up, damage sucks on small caliber, range sucks on larger, overall they can be beaten by everything else out there. Soooo....
Here's how I would fix them (after borrowing heavily from what others have suggested)
1) rename them. AC/2 = Light AC, AC/5 = Medium AC, AC/10 = Heavy AC, AC/20 = Assault AC (this goes for Light, Ultra, LB-X, and Rotary variants as well)
2) Up the damage output of the smaller calibers while deepening the ammo bins of the larger calibers. Light AC deals 4 pts, Medium AC 8, Heavy AC deals 12 (making it a freaking headcapper finally), and Assault AC gets 8 rounds per ton (making it actually worth using Ultra or specialty ammo).
3) everything else stays the same. This makes it possible to keep all old designs the exact same as they are now, but making them somewhat useful. Ranges stay the same. Heat stays the same. They don't become the weapon you just gotta have!!! But they become a weapon you might use (other than for flavor which I admit they have already in plenty)
Thoughts? Too powerful? the numbers can be easily tweaked and nudged. But I really think it's a step in the right direction. I for one am tired of looking at an AC/5 and thinking... "well it seems like a cool idea... but it seriously sucks!"