Realistically, ammo does not keep that well, just look at how badly poorly stored surplus ammo behaves in real life. Firearms probably reverted to cap and ball or flintlocks in areas where percussion cap tech is too advanced.
Yeah, I'd imagine breach-loaded pipeguns would become far more common than pistols, revolvers, or god-forbid, automatics.
Two questions:
Why should one care about realism on a game with talking chimps, psions and other mutants?
Without an industry to melt/forge/machine the guns how do you figure those would make a comeback?
Not being a gun nut, I figure the main issue with A/SA weapons and black powder is it lacks the power to reload/cock the gun, the solids can be cleaned.
So, either only revolvers, and bolt action rifles (are there still lever action ones being manufactured?) and shotguns would be in use, for those black powder was enough at the begining.
OR someone knows how, has the equipment and is manufacturing smokeless powder, a very involved process.
But that's only IF one wants to keep it real.