If you can't see any casting marks and it's lead, it's probably a modern muzzle loader swaged round ball. I've purchased and shot those round balls in calibers of .45, .50, .62. However those balls will not measure that size, they will be several thousands smaller, I don't remember how much on the .45 or .50, but fired from a rifle the ball is wrapped in cloth, so the fired ball many times doesn't show any rifling marks, but will show the imprint of the cloth patch. My .62 caliber rifle shoots a .615 ball that is patched using 13/1000 cotton, lubed with soap and water. On your ball, if there is a spru or anything indicating a mold mark, the ball will still be slightly smaller than the bore size, so like CBG says, to identify it, a measurement in 1/1000's of an inch is necessary.