I came up with a new gun design. This will probably be the system used for most of the military-style rifles you find for a while.
Charges per shot Attack bonus Damage Min. Avg. Max. Special
1 +2 1d8 + 8 9 12.5 16 Up to 16 shots/attack. No
attack may deal more damage
than the number of charges
used by that attack.
5 +0 2d6 + 4 6 11 16 Only 1 shot/attack.
The autofire setting will deal more damage more consistently, and its harder for enemies to evade, but at the price of over three times the ammo. Autofire should only be practical when you really need to hit something hard right now. I think x5 is a useful compromise between results and resources.
The minumum, average, and maximum damage are shown to better compare the two settings. I included an upper bound on x1 damage so that you can't use a handful of charges and still deal 1d8 + 8 damage--if you're low on ammo you're better off switching to x5. I guess the statistically smart strategy for the x1 setting might be to use 12-13 charges--since going for a full 16 means you'll "waste" ammo whenever you roll less than 8. All in all, the system seems much more balanced and useful now.