The En5ider patreon has several articles on guns from different eras.
https://www.patreon.com/ensider
https://www.patreon.com/ensider
I hope the OP ignores most of this thread, and just uses the weapons in the DMG and doesn't try to make them "realistic".
There's limitation on how 5e is and there's no equivalent of D20 Modern to compare against like there was in 3e. But generally I think reloading a gun could be seen as wasting an action. I realize that beyond 1 action to reload (and that's being generous), the only longer length of time that's worth bothering with is "out of combat", based on all the problems with tracking which round someone is on reloading.
I know the strong need for specialization and optimization makes the idea of a character mixing ranged combat and melee combat seem sub-optimal and totally undesirable, even though that's really was a thing where there was an opening salvo of gunshots and then everyone charged into melee with swords or bayonets or using muskets as clubs. And it might play up some of the more unusual combination weapons that existed like Axe-Guns or Rapier-Pistols, especially if they're the types of things that non-humans are way more into then Humans ever were.
It is about early firearms, maybe not really early firearms like Medieval Hand Cannons or Early Renaissance Matchlocks and Arquebuses, but when it's on muskets or things pirates might use then it's on the later Renaissance pushing into the Enlightenment, flintlocks and other such firearms. The Industrial Age or 1800's might be pushing farther than many peoples ideas on firearms in a campaign, but if it is then there's certainly breechloading, repeating rifles, cowboys shooting vampires, gentlemen or gentlewoman vs werewolves and the like.But, to be frank, much of this discussion assumes early firearms. Throw in paper cartridges and breech-loading and you have a whole different ballgame. And, why not go the extra step to make firearms specialization viable? Maybe the gunner loses one attack or bonus action for each reload. Maybe the guns are revolver-style where you can spend a whole action to reload the whole shebang for a few uninterrupted shots.
Of course the more one gets into the details, the more there's things like should Blunderbusses be a high damage/ short range weapon, or a cone area of effect that's resisted by a dex save.
The only time I ever used firearms was in a 2E campaign, where the PCs ended up with some wheellock pistols after a sojourn into Realmspace. I had them pretty simple... 1D4 damage, ignore armor at close range, 1 shot per round (considering that a 2E round was a full minute, reloading inside that space was reasonable). Just to simplify things, I had the PCs buy 'cartridges' (those paper/linen things they used back in the day) just like buying arrows or bolts, sold in packs of 20 just like them. Not incredibly accurate, but it did the job. Of course, different D&D systems will have to change it around (IIRC, a 5E round is 6 seconds, which is incredibly fast for reloading)…
I wonder if mythbusters ever tried the exploding barrel trick? Is gun powder packed tight enough in a barrel so it a BIG BANG or is just a BANG and you are ducking flying hoops?The problem with guns in a campaign world is not the guns.
It's the barrels of gunpowder that proves to be the real problem.