I agree with this. This is pretty much what I did in my Curse of Strahd campaign, where one of the PCs found the rifle in one of the tombs in the catacombs. I wanted it to be special and really damaging, so I basically made it mechanically the same as a wand of lightning bolts-- it did really good 8d6 damage on a hit, but also only had a certain number of "charges" per day.
Now granted, since I treated the rifle as a magic item I allowed it to be used multiple times per fight as you would any normal wand... but if you wanted guns to be a thing in the setting, do what Cosmic suggests. Make them one-shot items that do like 3d12 (with no ability modifiers added). And they can only be reloaded out of combat.
If you add in some roleplaying aspects as to why not everyone just overloads themselves with guns (exceedingly loud firing, only can holster one or two at a time, illegal to open carry, not thought of as a "gentleperson's weapon" etc... you don't have to worry about turning the game into a series of "shoot once, drop gun, draw another" repetitions for every PC.