US Cartridge, 20mm HPT, M54A1, M54E2
The cartridge is used for proof testing of 20mm weapons and barrels during manufacture and repair.
The cartridge is a solid steel projectile, under cut behind the ogive to control projectile weight, for developing high chamber pressure. The cartridge is identified by a stannic-stained (silvered) cartridge case.
Nothing else to see.
Collaborative Ordnance Data Repository