It was sold to him as a .50. Caliber SLAP round, Sabot Light Armor Penetrating. They’ve been discontinued for a couple of decades and are about $100 each if you can find them.
I’d never fire one in my .50 Serbu as the instructions specifically state firing any type of sabot will void the warranty.
L
I had never heard of that round so I googled it and the first hit said emphatically that the round was for machine guns only.
That would be warning enough.
If the round was too stout for military rifles of that vintage, heavy as they were.....