They had a class on black powder fire arms at the Renaissance Fiare, and it was fairly enlightening.
Loading a gun using a powder horn was sort of haphazard, as the measure of powder was irregular. You were going by feel.
A later development, which we saw in the 1500s, were "cartridges". These weren't the brass cased bullets we see today, but pre-measured portions of powder in individual containers. These gave better consistency to the powder charge used, but posed a separate problem: They were vulnerable, individually, to sparks and fire. And, of course, firing the weapon tended to spray hot sparks over the shooter from the primer pan.
So if you had a bandolier of power cartridges over your shoulder, you'd tend to hold the weapon away from your body to keep them from exploding in your face. That made aiming difficult.
Also note that the weapons of the 1500s were smooth bore. The old saying, "Don't fire until you see the whites of their eyes", made famous during the American Revolution, was actually good advice. You at least had a chance to hit your target if they were under 10 yards away. Otherwise the weapons were useless, except in massed fire at a massed enemy.
A gunner of the time was considered an expert if he could fire three times in a minute. That means two full-round actions plus a move action to reload.
You'd be dealing with matchlocks or wheel locks rather than flintlocks, of course.
A match-lock used a smouldering piece of rope or punk ("slow match") to ignite the powder. There wasn't a hammer, per se, but instead the trigger would rotate the match arm around to touch the primer pan.
Another important piece of advice for gunners was "Keep your powder dry". Black powder is ruined if it gets wet. You can't even dry it out, it's gone. That means that weather effects become a lot more important for the game (There's a table in the DMG.)
You may note that the heroes in Andre' Dumas famous tale, "The Three Musketeers", were famous for their swordplay, not their skill with the musket. In fact, I'm not sure if the tale even mentions them firing a musket, even once. The weapons weren't very reliable. They had impressive stopping power, which was why they were used, and a slug from massed fire could carry much father than any arrow, but it was impossible to actually hit a man-sized target at any range.
Pistol duels used to be fought at 20 paces (each man took 10 paces away from a start point). They frequently ended with neither man being hit.