One of the problems with the Renaissance Firearms is that they reload too fast. Historically experts could fire a musket about 3 times per minute. That roughly translates into taking 2 rounds to reload. But for the sake of record-keeping and ease of play at tables reload time could be reduced to 1 round.
I don't they should take bonus actions to reload as I want firearm combat to involve swords as necessary backup weapons, and having bayonets see use.
The same is true of crossbows -- but in 5E, for the sake of game-play, crossbows merely have the "loading" property which means you can't make multiple attacks in the same round.
But, a nice thing about requiring an action to reload, is that it encourages people to carry multiple loaded pistols, since drawing a new gun is much faster than reloading. Thus leading to the classic image of a pirate with a brace of four pistols.
At one point I toyed with including both in my campaign: the expensive "loading" guns in the DMG would be breech-loading flintlocks; but at character creation, all you could get would be old muzzle-loading matchlocks (or wheellocks or snaplocks or whatever) that cost 1/10 as much but required an action to reload. Eventually I ditched this idea because, in a raging fight, I didn't want to keep track of which NPCs had shot on the previous round. So instead I am making the cheap guns just do one smaller size of damage die, which puts them roughly on par with light/heavy crossbows.