Using my math, and by looking at the ruler I'm guessing it's around 11/16, which is close to .69 caliber. 10/16 is .625 which is to large for a .62 caliber, so my swag is it's probably about the right size for a Brown Bess. Anymore I forget most everything I learned over the years, so checked WIKI, and that says the Bess used a .69 caliber ball if that helps at all. I know they were a rather loose fit, they tore open the paper cartridge, dumped the powder down the bore, then rammed the ball, paper and all. There was a study done by the Brits, and it was discovered they got more hits over hard ground, than over plowed ground, so apparently they were bouncing them in. Someone else said that it was a mighty unfortunate fellow indeed, if he was being shot at on purpose 100 yards away, and suffered getting hit. With out exact measurements that's about the best I can do.