Cannon 21 is a cast iron 1/2-pounder or rabonette 3.5 feet in length from the base ring to the muzzle. The English proof mark on the 1st reinforce suggests manufacture by an English gunfounder.

The gun was cast in a two-part mold. The resulting seam is apparent on both reinforces and chase as well as the muzzle and mouldings. The mold consisted of two halves: one half (bottom) contained extensions for the trunnion and the other top half of the gun.

An unusual feature of this gun is a bevel cast on the right trunnion face. The bevel is not present on the left trunnion face, though it could possibly have been cast with one and face squared off after casting.