why do tanks have smooth bore guns

The cannon made the transition from smoothbore firing cannonballs to rifled firing shells in the 19th century. In more recent times anti-armour artillery, for example tank guns used for attacking tanks, is moving back to smoothbore. To reliably penetrate the thick armor of modern armored vehicles, a very long, thin kinetic-energy projectile is required. The longer the projectile is in relation to its diameter, the higher the spin rate must be to provide stability. Practical rifling can only stabilize projectiles of a limited length-to-diameter ratio, and these modern rounds are simply too long.

These rounds are instead formed into a dart shape, using fins for stabilization (see kinetic energy penetrator for information on how this works). With the fins for stability, rifling is no longer needed, and in fact the spin imparted by rifling would degrade the accuracy of a finned projectile. The first tank with a smoothbore gun was the Soviet T-62, introduced into service in 1961; today all main battle tanks except the British Challenger 2 and Indian Arjun MBT use smoothbore guns.

The Russian navy conducted experiments with large-caliber smoothbore naval guns, which were halted by budget cuts. The armour-piercing gun evolution has also shown up in small arms, particularly the now abandoned U. S. Advanced Combat Rifle (ACR) program. The ACR "rifles" used smoothbore barrels to fire single or multiple flechettes (tiny darts), rather than bullets, per pull of the trigger, to provide long range, flat trajectory, and armor-piercing abilities.

Just like kinetic-energy tank rounds, flechettes are too long and thin to be stabilized by rifling and perform best from a smoothbore barrel. The ACR program was abandoned due to reliability problems and poor terminal ballistics. Mortar barrels are typically muzzle-loading smoothbores. Since mortars fire bombs which are dropped down the barrel and must not be a tight fit, a smooth barrel is essential. The bombs are fin-stabilized.
In the 1960s smoothbore tank guns were developed by theP Pand later by the experimentalP P Pproject.

Based on their experience with the gun/missile system of theP, the Soviets produced theP B main battle tank, with an auto-loadedP, capable of firingP Pammunition as well as ATGMs. Similar guns continue to be used in the latestP P,P P, andP P PMBTs. The German companyP Pdeveloped a more conventionalP Pwhich does not fire missiles, adopted for theP, and later the U. S. P. The chief advantages of smoothbore designs are their greater suitability for fin stabilised ammunition and their greatly reduced barrel wear compared with rifled designs.

Much of the difference in operation between smoothbore and rifled guns shows in the type of secondary ammunition that they fire, with a smoothbore gun being ideal for firingP Prounds (although specially designed HEAT rounds can be fired from rifled guns) and rifling being necessary to fireP Prounds. Most modern MBTs now mount a smoothbore gun, with the BritishP PandP P Pbeing notable exceptions.