I've done this before (I actually have a full rebalance of all the weapons of Battletech that does not touch crit space, tonnage, and BV only minimally. I use it in Megamek especially).
BV is definitely not the way to go here. The problem with balancing via BV is that you're trying to use a second system to patch a broken first system. As such, it's easy to find and expose flaws with BV. It's better to correct the broken system in the first place--that is how the weapons balance and operate in reference to each other and within the mech design rules.
I took my inspiration directly from what is quoted numerous times about the Clans in the fiction--they load up on expendables for assaults because they expect a quick victory, and expendables supposedly hit harder. Well, in the current balance they largely don't, but in mine they actually do--at the cost of running out of ammo and other issues.
ACs:
First, take the optional rapid fire rule and make it function similar to how Ultra and MASC work now. Perma jam on 3, and since the AC's aren't made to fire sustained, next failure is 5, next is 8, etc.
AC2: 8/16/24/(32) range brackets, 2 damage, 0 heat, no min range. 70 shots per ton.
AC5: 6/13/21 range brackets, 5 damage, 1 heat, no min range. 28 shots per ton.
AC10: 5/11/18 range brackets, 10 damage, 2 heat, no min range, 14 shots per ton.
AC20: 3/7/12 range brackets, 20 damage, 4 heat, no min range, 7 shots per ton.
In my extensive playtesting experience, this helps AC in comparison to their energy contemporaries tremendously and gives good reason to use them.
AC2: It's still a relatively weak niche weapon, but in IntroTech it at least has a unique niche. Removing the silly min range as well as heat helps to pair it with other weapons where it can at least be properly supplementary to them (Blackjack and Jagermech, as examples).
PPC vs AC5: The AC5 has a longer long range that can bracket well with LRMs and the lack of min range helps it stack as a heat efficient pairing to medium lasers and other small weapons. This helps make designs that utilize it in such a mix like the Zeus and Shadow Hawk make a lot more sense. The PPC is still both more efficient and scarier, even with the occasional rapid fire from the AC5, but the AC5 is much more flexible. This helps offset the ammo kabooms and running out of said ammo.
AC10 vs Large Laser: The AC10 packs a bigger bunch out to ranges that can pair alongside the PPC (Banshee 3S as one possibility where this makes sense). The minor heat drop, the occasional rapid fire, and the slight range increase are really all this needed to be a bit more competitive with the large laser (one of them still goes boom and runs out of ammo, the other doesn't).
AC20 vs Medium Lasers: Even with the heat adjustment medium lasers are still more efficient within the first 10 heatsink range, but beyond that the AC20 starts to catch up. To counteract the kabooming and such, the AC20 likewise gets a longer (unoccupied in 3025!) range bracket for mid and long. Also, even if it's still not winning in terms of actual efficiency, the possibility that the AC20 can potentially double tap on you puts the proper fear of our based RNG god into people.
These changes then carry over into Ultras, with Ultra's getting a touch more range at the expense of tonnage and heat. Ultra mode simply changes where the fail on 2 is a jam you can clear. I think my IS Ultras look something like:
UAC2: 8/17/27/(36), 2 damage, 0 heat
UAC5: 7/14/21(28), 5 damage, 1 heat
UAC10: 6/12/18, 10 damage, 3 heat
UAC20: 4/8/12, 20 damage, 6 heat
Or something like that, I don't have it right in front of me.
Anyhow, I can extrapolate more on this. Or I might already have somewhere else.