Don't get me wrong, I think D&D works fine with guns, up to a point. I'll quite happily use 5E for steampunk settings that use firearms and it's dandy. The firearms I use don't tend to be that much different than the missile weapons already in the game though. I up the damage a little, maybe monkey with the ranges, but I'm not changing much. I'm not aiming for deadly firearms though, which is the main reason I don't have any issues IMO.
When this topic came up I did a bit of googling, and 19th century rifles have about the same effective range as longbows with a few exceptions (such as the Sharps rifle which was the equivalent of a sniper rifle). But your basic Winchester? About the same. Pistols? Close to shortbows. Add in the fact that they used black powder and I don't even see that much of a need for dramatically increased damage.
But it is all make believe after all. If we assume that in a world were magic is real there can be armor that stops bullets, I don't see much need for adjustment. I might come up with a feat for speed firing or similar, but I'd probably just use bows and maybe increase the damage die one size and be done with it. In any case, most of what I hear is "they wouldn't work because getting shot once is always fatal" with details on what the rules would actually be.