Just chiming in briefly to say that I think this is an elegant and very workable solution to the problem of balancing some measure of verisimilitude against game balance in regards to firearms.
As a DM, I use this very bit of fluff fairly often myself in regards to warriors wielding heavy weapons. If they are fighting only a single opponent, I've been known to coalesce multiple hits on a full-attack into one big, bone-shattering strike landed amongst the various feints and parries.
Regarding the issue of multiple misfires, I'd say that it's easily addressed by simply stating that a given gun can only "misfire" once in a single round of attacking, and that a "misfire" doesn't preclude hitting and dealing damage with the gun. Seems fair to me.