APFSDS

History

The spin from standard rifling decreases the performance of these rounds (rifling adds friction and converts some of the linear kinetic energy to rotational kinetic energy, thus decreasing the round's velocity, range and impact energy). A very high rotation on a fin-stabilized projectile can also increase aerodynamic drag, further reducing impact velocity. For these reasons APFSDS projectiles are generally fired from smoothbore guns, a practice that has been taken up for tank guns by western and eastern blocs. Nevertheless, in the early development of APFSDS ammunition, existing rifled barrel cannons were used, (and are still in use), such as the 105 mm M68/M68E1 cannon mounted on the M60/A1/A3 main battle tank or the British 120 mm Royal Ordnance L30 of the Challenger 2 tank. To reduce the spin rate when using a rifled barrel, a "slip obturator" is incorporated that allows the high-pressure propellant gases to seal, yet not transfer the total spin rate of the rifling into the projectile. The projectile still exits the barrel with some residual spinning, but at an acceptably low rate. In addition, some spin rate is beneficial to a fin-stabilized projectile, averaging out aerodynamic imbalances and improving accuracy. Even smooth-bore fired APFSDS projectiles incorporate fins that are slightly canted to provide some spin during flight, and very-low-twist rifled barrels have also been developed to fire APFSDS ammunition.