To get the best kind of guns, go to Antwerp and invest in the shipyard until it's at 1000. Now not only will you be able to build Full-Rigged Ships, and Frigates, but also if you go there at 4:00 AM you can buy the Carronade, a kind of gun with the fire power of the cannon and the range of the Culverin. It costs 600 gold pieces per gun.