I realise that "projectile" can normally cover anything from a spear to an ICBM, but in the technical terminology for shells, an armor-piercing shell is said to consist of the "sabot", which is launched from the gun carrying the "projectile". The "sabot" falls off and the "projectile" carries on and pierces the armor.

See following quote from reference given.

Tank and antitank guns have become a very special field. At present, they are typically 120 mm (5 inch) and are equipped with thermal sights and laser rangefinders. A typical antitank projectile is a solid rod of a dense metal (tungsten or uranium) fired at extremely high velocity. To get the velocity, the projectile is commonly smaller in diameter than the gun bore, and is carried by a 'sabot' which fills the bore and drops off when the projectile leaves the barrel. The velocity is so high that rifling no longer works, and the projectile is fin-stabilized (APFSDS: armor-piercing fin-stabilized discarding sabot) (Flьgelstabilsiertes Treibkдfiggeschoss). On impact, the projectile punches a hole through the tank armor, releasing hot fragments that bounce around inside. Other anti-tank projectiles include HEAT (high-explosive anti-tank) with a shaped charge that produces a penetrating jet, and HESH (high-explosive squash-head) which is not intended to penetrate, but to make a piece of armor flake off at high speed (spall) inside the tank.