I'm allowing personal black powder weapons in this game. It just wouldn't be a pirate game without guns. Cannon, however, are getting left out. I didn't find any cannon rules (let alone ones I liked) aside from rules for field artillery for mass land combat. Maybe I didn't look hard enough, but I've got no problem allowing pistols and blunderbusses (blunderbi?) while not allowing the big guns.
I have stats for pistols, dragons (pistol blunderbi), muskets, blunderbi (okay, I'll stop now), hand held bombs and large placed bombs. Gamma World 4 and d20 Past were my primary sources of information/inspiration for this.
Pistol -- damage 2d4 -- rate 1/3 -- range 20/40/60 -- price 100gp -- enc. 40cn
Dragon -- damage 3d4 -- rate 1/3 -- range 10/20/30 -- price 150gp -- enc. 60cn
Musket -- damage 2d8 -- rate 1/3 -- range 40/80/120 -- price 300gp -- enc. 100cn
Blunderbuss -- damage 3d6 -- rate 1/3 -- range 30/60/90 -- price 400gp -- enc. 140cn
Hand Bomb -- damage 3d6 (10' radius) -- rate 1/2 -- range 10/20/30 -- price 50gp -- enc. 50cn
Large Bomb -- damage 5d6 (20' radius) -- placed only -- price 250gp -- enc. 250cn
Powder and Shot (10 rounds), any type -- price 25gp -- enc. 25cn
These weapons misfire on a natural 1 on the hit roll, resulting in a) an automatic miss, b) necessity to clean the weapon (1 turn) before use, and c) Save vs. Wands or take 1 die of damage of the weapon's die type.
Any of the four gun types can be used as a club (1d4 damage) and the musket only can have a bayonet for 1d6 damage.