Traditionally, shells were given stability in flight from the rifling of the gun barrel, which imparts a spin to the round. Up to a certain limit this is effective, but once the projectile's length is more than six or seven times its diameter, rifling becomes less effective.
Adding fins like the fletching of an arrow to the base gives the round stability. The spin from rifling decreases the effective penetration of these rounds (rifling diverts some of the linear kinetic energy to rotational kinetic energy, thus decreasing the round's velocity and impact energy) and so they are generally fired from smoothbore guns, a practice that has been taken up by China, Israel, France, South Korea, Germany, Russia, and the United States in their tanks. Another reason for the use of smoothbore guns is that shaped charge HEAT munitions lose much of their effect to rotation. APFSDS can still be fired from rifled guns but the sabot is of a modified design incorporating bearings to isolate the spin of the sabot in the barrel from the round itself, so far as practicable.
Rifled guns have been kept in use by some nations (the UK and India, for example) because they are able to fire other ammunition such as HESH rounds with greater accuracy. However, the rifling wears down under regular APFSDS use and requires more maintenance. For these reasons a
British Challenger 2 tank was modified under the Challenger Lethality Improvement Programme to mount a Rheinmetall 120mm
smoothbore gun. However this programme has now been discontinued because of the impossibility of stowing an adequate quantity of the new fixed (one-piece) ammunition without replacing the powerpack to create additional hull volume.
 was initially the main design of the kinetic energy (KE) penetrator. The logical progression was ...){kind=link}