Dave (SG500) you are close enough. In the U.S., there are only four types of flat nose projectiles, antisubmarine, burning smoke, canister, and proof. This is a high pressure test proof projectile for a 105mm tank gun for a RAKE projectile (Rocket Assisted Kinetic Energy). The idea was to have a heavy metal dart with a rocket motor on the back to increase velocity. Since the 105mm Gun is loaded to max pressure, they need to work up the powder charge with the exact shape of the projectile in the case. This projo is solid aluminum, the bottom part machined in the shape of the rocket motor. I will add photos of motor remnants.
SG500 over to you.