It is a U.S. Navy 5 inch 54 cal High-Frag projectile bottom-half. It uses a plastic cup-shaped piece that acts as a rotating band, which locks on to the groove at the base. The plastic piece gains some traction from all the knurling on the boat-tail. The idea behind these, was that the explosive was pre-manufactured into a plastic encapsulated sausage-looking cartridge, which was placed inside the base. The upper-half of the projectile was then pressed down over the top of the base piece. The two were supposed to have a precise interference-fit. We produced these projectiles at Norris Industries in Vernon California in 1989. I actually programmed one of the manufacturing steps that machined the bottom of the base.
These were manufactured from a special Hi-Frag alloy. I heard later from a co-worker, that the projectiles were a failure and would break into the two original pieces in mid-air. We produced thousands of them, more than just a pilot lot. I have a number of them that I pulled out of scrap, that were out of tolerance when inspected. I have both halves. I do not know what the model number of the projectile was. I've never seen it in an ammo manual.
Your photos are very deceptive. The boat-tail which is very visible in a good profile photo, is almost unrecognizable in the fish-eye photo. I recognized it by the reduced diameter mating section and the groove and knurling on the base. It is shown in a post of mine from 2011.
http://www.bocn.co.uk/vbforum/threa...-from-the-O-Brien-room?highlight=O'Brien+room
second post down, middle of the left-most photo, the one with the red bottom half.
Your projo bottom was most likely fired and came apart in mid-air. The plastic Obturator piece breaks off when the projectile leaves the muzzle, thus no rotating band.
Can you tell me what the Mk. and Mod numbers are that are inked on the projectile, along with the lot number and other labeling data? I need to label my examples. I can read 5" 54 on the body in your first photo.
