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A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.

A smooth-bore, cast-iron ship's cannon, from the Grand Turk, a replica of a mid-18th century three-masted frigate

A shotgun fires multiple, round shot; firing out of a rifled barrel would impart centrifugal forces that result a doughnut-shaped pattern of shot (with a high projectile density on the periphery, and a low projectile density in the interior). While this may be acceptable at close ranges (some spreader chokes are rifled to produce wide patterns at close range) this is not desirable at longer ranges, where a tight, consistent pattern is required to improve accuracy.[4]

Another smoothbore weapon in use today is the 37-mm riot gun, which fires non-lethal munitions like rubber bullets and teargas at short range at crowds, where a high degree of accuracy is not required.[5]

The Steyr IWS 2000 anti-tank rifle is smoothbore. This can help accelerate projectiles and increase ballistic effectiveness. The projectile is a 15.2 mm fin-stabilized discarding-sabot type with armor-piercing capability which the IWS 2000 was specifically designed to fire. It contains a dart-shaped penetrator of either tungsten carbide or depleted uranium, capable of piercing 40

Another smoothbore weapon in use today is the 37-mm riot gun, which fires non-lethal munitions like rubber bullets and teargas at short range at crowds, where a high degree of accuracy is not required.[5]

The Steyr IWS 2000 anti-tank rifle is smoothbore. This can help accelerate projectiles and increase ballistic effectiveness. The projectile is a 15.2 mm fin-stabilized discarding-sabot type with armor-piercing capability which the IWS 2000 was specifically designed to fire. It contains a dart-shaped penetrator of either tungsten carbide or depleted uranium, capable of piercing 40 mm of rolled homogeneous armor at a range of 1,000 m, and causing secondary fragmentation.

The cannon made the transition from smoothbore firing cannonballs to rifled firing shells in the 19th century. However, to reliably penetrate the thick armor of modern armored vehicles many modern tank guns have moved back to smoothbore. These fire a very long, thin kinetic-energy projectile, too long in relation to its diameter to develop the necessary spin rate through rifling. Instead, kinetic energy rounds are produced as fin-stabilized darts. Not only does this reduce the time and expense of producing rifling barrels, it also reduces the need for replacement due to barrel wear.

The first tank with a smoothbore gun was the Soviet T-62, introduced into service in 1961. Today all main battle tanks field them except the British Challenger 2 and Indian Arjun MBT. While the 73 mm gun of the early Soviet infantry fighting vehicles BMP-1 and BMD-1 was a smoothbore, their more recent successors BMP-3 and BMD-4 use a rifled 100 mm gun. 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 The first tank with a smoothbore gun was the Soviet T-62, introduced into service in 1961. Today all main battle tanks field them except the British Challenger 2 and Indian Arjun MBT. While the 73 mm gun of the early Soviet infantry fighting vehicles BMP-1 and BMD-1 was a smoothbore, their more recent successors BMP-3 and BMD-4 use a rifled 100 mm gun. 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 that are dropped down the barrel and must not be a tight fit, a smooth barrel is essential. The bombs are fin-stabilized.