Suddenly a game that privileges ease of play over realism is bogged down in a mire of special properties, edge cases, and bolted-on house rules that seem at odds with the base system.
Lamentations of the Flame Princess is a pretty good example of this effect in action. LotFP is a game content to abstract melee options. It doesn't have a big detailed chart of everything you can use to bludgeon or stab someone to death with in the game; instead, weapons are ranked great, medium, minor, or small, and you're left to fluff them accordingly.
(There are a few weird outliers, like the cestus, polearm, and spear which have some special-case rules, but generally things are kept simple and consistent rather than realistic and detailed.)
And then you get to the firearms appendix and all that simplicity and consistency flies out the window. Now you've got bullet-pointed lists of special rules for firing mechanisms (with asterisked exceptions), gun and barrel types, and any firearm accessories with mechanical add-ons (such as apostles) that you're bringing to the party.
My own firearm rules back when I was playing Labyrinth Lord had moments of being equally as convoluted and contrary to the free-wheeling spirit of the rules. At various points I had bespoke rules about range and reloading based on some way-too-intensive research, exploding damage dice rules, etc. It was a mess and it added nothing good to my games.
Instead of coming up with new cruft to add to the game, I should have taken inspiration from Erik Jensen and just used the rules for ranged weapons that already exist in the game. At the level of abstraction that most old-school D&D games default to, you're just better off using the stats of bows or crossbows and reskinning the fictional aesthetics of the weapon than detailing all sorts of new rules to make it "realistic." John Bell gets it. Brian Mathers gets it.
5e D&D has somewhat of the opposite problem. Firearm rules are buried in an optional section of the Dungeon Master's Guide (267-268). Generally, the rules are pretty simple: the black powder firearms follow the rules already extant for crossbows, except they do a bit more damage. More modern firearms also have similarly efficient rules for their use. No problem, right?
Well, no, not exactly. Since they aren't part of the default game assumptions, they don't really interact well with things like special abilities or feats. If you use them as-is, there's no real reason to pick a firearm over a crossbow; if you start house ruling to make similar feats available for firearms, there's no reason to use anything but a firearm because their damage is just plain better.
Oddly, the solution to 5e's problem is the same as the solution to the old-school problem outlined above: just use the stats for crossbows, since they are already integrated into the game, and refluff the descriptive fiction as black powder firearms. A heavy crossbow could certainly be ye olde arquebus, a light crossbow could be ye olde musket, and the hand crossbow could be ye olde pistol. You don't have to invent rules about which class is proficient with which; just look to see which crossbows they can already use with proficiency and apply it to firearms as well. You don't have to come up with new feats; change the wording to Crossbow Expert and you're good to go.
