Smoothbore
In the 1960s smoothbore tank guns were developed by the Soviet Union and later by the experimental US–FRG MBT-70 project. Based on their experience with the gun/missile system of the BMP-1, the Soviets produced the T-64B main battle tank, with an auto-loaded 125 mm smoothbore high-velocity tank gun, capable of firing APFSDS ammunition as well as ATGMs. Similar guns continue to be used in the latest Russian T-90, Ukrainian T-84, and Serbian M-84AS MBTs. The German company Rheinmetall developed a more conventional 120 mm smoothbore tank gun which does not fire missiles, adopted for the Leopard 2, and later the U.S. M1 Abrams. 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 firing HEAT rounds (although specially designed HEAT rounds can be fired from rifled guns) and rifling being necessary to fire HESH rounds.
Most modern MBTs now mount a smoothbore gun, with the British Challenger 2 and Indian Arjun being notable exceptions.
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