Currently, there's a bit of an odd discrepancy between the small and large bore autocannons. That is, the AC2s and AC5s fire similar to fluff (a change from Mechwarrior 3 and 4, but similar to 2). 10s and 20s, on the other hand, act more like MW4, with single shots and long waits on reloads, and not so much similar to the fluff (wherein AC10/20 blasts are usually described as stitching a burst of rounds over a target). I think a change towards the fluff style would both make them more interesting and add to weapon variety.
AC 20s are described in fluff as having a rate of fire of 1-10 rounds per CBT round, depending on the manufacturer. I would call a happy medium 5 or so, myself. As such, I think that a rate of fire of a round a second with a max of 5 seconds firing before needing cooldown. Obviously, damage would need to rebalanced as well. Rather than the current AoE effect 20s have, damage would be single area (the damage over multiple areas more likely coming from the likely spread of sustained fire). Heavy Gauss at close range presently does 3100 points of damage per hit at close range (with the ability to be used as a close range and poptart weapon). I would propose about 500 damage per hit to a single area from the AC20 rounds, thus equaling 2500 damage total over the full 5 second period. UACs would fire double the rounds in the same time, bringing them to 5000 damage over 5 seconds. Considering none of that is guaranteed to hit the same area, and one has to expose themself for the full time period to be able to hit a target with all of those rounds, I feel that's balanced towards the h-gauss ability to hit a single area for 3100 guaranteed damage in just a moment of self-exposure.
I suppose AC10s could have roughly double the fire rate at a little under 1/4 damage, equalling 2 rounds a second at about 110 damage a hit.. or about 1100 damage over the same 5 second period (with longer range and better projectile speed).
Changing to this mode of fire, in addition to making for a more interesting, brawler-like weapon, would also make LBXs a more distinguished and unique weapon.. leaving the role of single-shot, spread-area damage to them.
To try to represent the different rates of fire for each AC in typed out 'sound effects':
AC2: Cha-dadadadadadadada
AC5: Chatchatchatchatchatchatchat
AC10: Boom-boom-boom-boom-boom-boom
AC20: Ba-boom, ba-boom, ba-boom, ba-boom
(You know you love it)
Thoughts?