I feel like if you just used the same stats as crossbows, it would work out better than this. We have crossbows in 3 sizes, so that should do it for pistol, rifle and musket. But crossbows are also known for reloading slowly, not requiring a great amount of skill to use it and packing a similar punch to early firearms. The only real differences mechanically would be that you have to carry more weight to have a good amount of crossbow ammo, but if you are using gunpowder then you have to keep it try or it just won't lit.... also, if you get engulfed in fire, then any gunpowder you have might go off-- honestly, those last two restrictions might make it a very bad option for adventurers.
Then again-- I think the mechanical differences between a lot of weapons is entirely unjustified. I think a lot of it is just there because if you add another weapon to the weapons table, people feel the need to give it different properties from every other weapon on the table even though all you have to work with is a die size and crit range to work with unless you want to add some sort of special rules to make it even more complicated. The designers feeling compelled to give every weapon unique stats is why we have rapier as the god weapon of 5E that pretty much by itself has utterly screwed up combat balance for the entire edition and is a not insignificant contributor towards Dex being the god stat.
I guess what I am trying to get at is simply.... instead of trying to make a range of weapons, in which some are just so bad that one would never want to use them except as a last resort even though they are so rare that they would never be your last resort weapon.... while others are just so good that any character is going to want to seek it out and use it as it instantly increases their damage output by 5-20%....
Just use the already existing array of balanced weapon stats. It is all abstracting a bunch of factors that the game can't really account for well down to a simple die roll.
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