In about 1970, a USAF compatriot of mine gave me a small collection of ACW bullets from his farm in VA. One of them appears to be about .54-56 cal, two definite, possibly a third top grease-grooves, and has what appears to be a solid washer-on-post protruding from the nose of the short bullet. It has a small cavity in the base, perhaps about .22" dia. and about .25" deep. "Gary" said that this is the only one like it he'd ever found on the farm and had never seen another like it. Nor have I. Anyone have knowledge or a reasoned guess? A "bore-cleaning" round? TIA.