Gunsil is correct... it is too small to be a Canister-ammo ball or grapsehot ball. Besides the fact that there were no iron 1/2-inch canister balls (nor grapeshot balls), your measurement of 1/2-inch seems to include the thick rust-crust still on the ball, so its actual metal diameter is less than 1/2-inch. Sorry to have to give a disappointing report, but it is a ball-bearing.