Are they lead?
May be leftovers from soldiers carving their bullets, which was popular. Also, judging from their size, he may have carved larger bullets down to a smaller caliber, and that's the leavings.
It is also possible that is part of a load of canister shot, shot from a cannon. Technically, canister was a mess of round balls, like a big shotgun shell, but they could and would fire anything. Shudder