Johnny Angel
Explorer
Flintlock pistols were in fact carried loaded, needing only action on the hammer with the thumb to make them ready to fire. You didn't reload them in the middle of battle, you dropped them as soon as they were discharged, hoping you'd live to pick them up later, and draw another from your 'brace of pistols'. So, nearly the same effect was available in real life.So, if D&D had rules for muzzle-loading flintlocks, and there was a feat allowing you to fire flintlock A in your right hand (main action), and flintlock B in your left hand (bonus action), *every round*, would you have characters do that? You don't care that reloading a flintlock IRL takes a free hand and several steps (pour powder, load wadding, load bullet, ram tight, prime pan, reset striker)?
Is this any different from one-handed crossbows which draw their own bowstring taut enough to hurl a bolt, AND place a new bolt in the groove, instantly, without the use of a free hand?
If your campaign allows for the flavor of mechanical wizardry, there are plausible ways to imagineer what is in effect a self-reloading hand crossbow - or at least a quarrel launcher. Being able to pluck back the actual string to feed off the tension in an actual bow seems too Rube Goldberg compared to the more plausible action principle of a bolt powered by a spring that has to be wound or a pneumatic system that has to be pumped. I've also never liked the notion of having a crossbow with a sort of magazine, though I am told there is historical precedent for this. I like the drum concept better for this.
But I do agree with you that going through all the hand-waving about how a thing would be mechanically possible seems like just bending over backwards to deny firearms a place in your campaign. However, I myself would actually use both - comically complicated gnomecraft items that expand the functionality of slings or crossbows, plus the relatively simpler real-world technology of flintlocks and even revolvers.