I tried over on the BOCN forums without any guesses so I figured it would be better over here where the serious cartridge collectors congregate.
This round is a blackened dummy with a blind primer pocket, headstamped FA 62. The overall length is exactly the same as a standard .50 BMG round but the case itself is longer, 107.7mm vs. 99mm. It simply has a longer case and a shorter bullet. The bullet is .402" in diameter (10.2mm).
I’m certain this isn’t some sort of a homebuilt wildcat. It came from a group of 13 identical specimens, all with the same headstamp and blackened finish. The finish looks like many other GI blackened dummy rounds of that period. Note the slightly distressed and smeared case head. The other rounds also had similar markings.
My assumption is a type of high velocity experimental of some sort. I’ve seen lots of odd ducks like .20/.50s, .50/30s, and others but not one like this. Was it intended for single shot velocity tests or automatic gun system? Anyone recognize it or able to fill in the historical blanks? Thanks.