Armor-piercing, fin-stabilized, discarding-sabot
An armor-piercing, fin-stabilized, discarding sabot (APFSDS) projectile, fired from a 120mm gun, is the primary anti-tank weapon of modern tanks. The single exception among tank-manufacturing countries is Britain, which prefers the high explosive squash head projectile.
The "armor-piercing" is done by a long rod made of a hard, dense metal such as depleted uranium or tungsten. Much like an arrow, the rod has fins for stabilization, rather than being spin-stabilized as are most artillery shells and small arms.
It travels at extremely high speed for a cannon projectile, and it achieves this speed through the discarding sabot technique. A typical APFSDS complete shell, ready for firing, is of 120mm caliber (i.e., the projectile has a 120mm diameter). The dart-like projectile, however, is only 40mm in diameter. For firing, it is surrounded by a two halves of a lightweight plastic and metal jacket called a sabot, which falls away after the projectile emerges from the muzzle of the gun. Having the sabot allows the firing energy of a 120mm charge to be applied to the 40mm dart, energy that could not be imparted onto a 40mm diameter base.