Try a .672 RB. You can buy them at Track of the Wolf, and other suppliers. They also have .675, and .678 diameter balls.
YOu want a ball that is .015 to .020" smaller in diameter than your bore diameter, and you will use a patch that is .015 to .020" thick. Remember that patching material will compress to 1/2 the dry thickness, when you lube it, either with a wet lube, or a oil lube. Those large diameter balls will upset fairly well under a good size powder charge to fill that barrel.
Don't expect much in the way of accuracy from a smoothbore pistol. They usually don't have any sights at all, or just a small front sight. The barrels are in the 9-12 inch length range, so you can't get much help from the barrel in pointing at distant targets. This is a 15 yard gun, basically, unless you are shooting at a target the size of a man. Then you can double that range.
These guns were carried in pouch holsters on saddles, in pairs, and used to supplement a sword in fighting. The pistol would be fired at a rushing cavalry soldier, and if hit, would take him off his horse. That would upset the horse, and in turn the chaos would block the patch for other rushing cavalry troops. The smoke from the pistols would also obscure the view of the soldiers being attacked, giving them time to shoot several such pistols at the rushing enemy before resorting to their swords. Any kind of hit with these large balls would usually result in a fatality, simply because the balls carried with them a lot of road dust and dirt, which infected wounds. The ball might pass through, but the patient more often than not would die from infections. There was NO penicillin or any other anti-bacteria drugs in those days. Its not amazing how many men died of wounds, in those days: its amazing at how many survived wounds.
