My general view is that a D&D character using a double-barreled shotgun or a revolver should still only roll one attack per round, with the number of bullets/shells used being a bonus on the attack roll. So, firing one shot from a six-shooter would be worth a +1, unloading everything would be a +6. Counting shots would be too annoying; after all, I've been considering rolling for number of arrows used by an archer at the end of a combat, rather than having the player deduct arrows as used.
So my solution is that there are only three kinds of attacks with a multi-shot gun:
- Single shot, exactly as it sounds.
- Half your shots, bonus = half the gun's capacity, round down (+3 for a six-shooter.)
- All your shots, bonus = gun's full capacity (+6 for a six-shooter.)
Each of these takes one round. After one attack using all your rounds or two attacks over two minutes using less than the gun's full capacity, you have to reload to use the gun again, which takes a full minute.
The reason why this matters is because I was thinking: "What if I were running a pseudo-Western campaign based on Stephen King's Dark Tower series crossed with El Topo?" And it occurred to me that there would be a Fighter variant called "Gunslinger" which would have the reloading trick King described in The Gunslinger. Instead of having to reload immediately, Gunslingers can keep firing full loads every combat round, for a number of rounds equal to level. Thus, a 1st level Gunslinger would be about the same as any Fighter with a gun, but a 2nd level Gunslinger could hold enough bullets in the off-hand to keep firing for two rounds, reloading in the middle of firing. (Also, of course, Gunslingers would shoot multiple normal 1 HD opponents every round, but only when using the gun. A 9th level Gunslinger would be a horrible death machine in a town of ordinary citizens.)