Attached photographs show sub-caliber devices utilized by U.S. Navy and U.S. Army for cheap practice and artillery training.
The devices used modified U.S. Kragactions and fired the ammunition mentioned in rcathey's (posted) DCM/D.O.D. letter.
The Naval device bolted to the side of a deck gun and was fired parallel to the artillery gun bore. The Army device (excavated on Bannerman's Island) screwed into the breech threads of larger artillery pieces to fire through the artillery bore.
Attachment 94495Attachment 94496