The mech with the most kills in my lance is a King Crab with 2 AC/20+++. Called shots are terrifying. 25 total shots, so 12 for each gun with one spare. Has some MLs to supplement, but nothing shrugs off 2 120-point hits to the same location. Sometimes I’ll only fire 1 AC/20, either for heat purposes or because 2 would be overkill, so I have yet to run out of ammo in a mission.I've never much enjoyed using AC/20s. If a weapon needs ammo, I want enough for at least ten shots per battle. For AC/20 that's a few extra tons of ammo per weapon and the cannons themselves are not light! I usually keep to AC/2 and AC/5s for long-range firepower, then using banks of SRMs and medium lasers for close-in firepower.
Granted, if I could find multiple Gauss Rifle+ and the ammo for them, I’d use that instead.