We’ve mentioned before, in the very early days of the blog, the difference between HEAT rounds and Sabot. The M256 120mm main gun of the M1A2 Abrams tank uses semi-combustible ammunition.
Here’s a cutaway image of a HEAT round.
And here’s the base stubs left after firing, taken from inside the turret of an M1.
You need a couple of inches of length on the stub in order to provide a good seal at the breechblock of the gun.