Handgonne - Triple dice of damage within range of 30ft. Point blank shot feat does not apply. Handgonnes require 10 rounds to reload. Alternatively - Make a strength check as a full action. Each time you roll subtract the number from 100. When you reach zero or less, the gun is loaded. So effectively, roll 10 or greater each round in order to go under the 10 round requirement. Rapid reload feat gives you a +4 to these strength checks. Reloading a handgonne requires care in setting the wick, packing in the powder to a density at which it can ignite properly with enough of a bang to launch the rather heavy bullet (bullet might weigh anywhere from 1/2 to 2lbs., methinks - it's usually metal or stone of any hard material you can find).
Powder is contained in a caske. If it's raining, you can't use this weapon. Drying the weapon takes 4d8 minutes. Storing the powder is easy - rennaissance tech probably assumes that you can purchase water tight barrels/caskes. Powder horns can get wet, however nobody travels around with multiple horns of powder (better to take a whole caske with you). A caske contains maybe 40 horns worth of powder. Reloading a handgonne requires a full horn (or half of one; I dunno, GM discretion - point is, it takes a lot of powder; these are the lowest tech firearms you can get).
For balance - handgonnes should do the most damage of any firearm, but basically don't work beyond 70ft. without a natural 20 (and even if they hit, they will do minimum damage - even on a confirmed crit (which is another natural 20, so 1 in 400 chance) it will do minimum damage for the critical hit's additional dice). Damage is rolled regularly between ranges of 31 - 70ft. and -6 to attack without sacrificing move action to aim. Within 10ft., no aiming required and a hit is gauranteed (unless enemy does a standard action to avoid, in which case they get to save on reflex vs. DC 20).
Standard action ignites the whick, and the weapon fires at -10 your initiative (at negative 0 initiative, the weapon obviously fires last in the turn order). You can shorten the wick for less initiative count, but this increases the chance of misfire (use ordinary misfire rules).
Alternatively, heat a metal rod with a forge and shove it into the back of the barrel. This increases the chance of misfire considerably, but shortens shooting to a move action and you must have a way to prop the barrel up and aim it accurately enough to fire. Essentially, this transforms it into an artillery implement with terrible range (bombards, also tech of the times, are more popular for their much greater range, and a parabolic flight that favours much heavier bullets - basically something similar to cannon balls, I guess). This is great for using the gun as a siege tool.
Matchlock - Arquebus and other type firearms came around in the early modern period (late 15th century and onwards until 17th century). Kind of like handgonne, but load faster, maybe. I don't really understand the mechanics behind this one. The real difference is that they can be propped on the shoulder and helped warfare evolve into different tactics, such as pike and powder formation.
Flintlock - Muskets and such. These are the ideal firearm, because they are much more accurate. Not nearly as accurate as guns today, but accurate nonetheless. Still prone to misfire, and such.
Judging by the pistol, who's to say the smooth bore longarm isn't equally accurate (rifling is more about range, not accuracy, so I've heard)? It could be that the pistol is rifled, but maybe that's unlikely. I don't really know.
Referance #4 - handgonne!
From that video alone, middle ages are just so much cooler.