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

History of firearms and rifling

Early firearms had smooth barrels, and fired projectiles with no significant spin. These projectiles had to have stable shapes, such as finned arrows or spheres, to minimize tumbling during flight. However, spherical bullets do tend to rotate randomly during flight, and the Magnus effect means that even a relatively smooth sphere will curve when rotating on any axis not parallel to the direction of travel (see knuckleball for an example of intentional random tumbling.)

A rifled barrel, having spiral grooves or polygonal rifling, imparts a spin to the projectile which stabilizes it and prevents it from tumbling. This does two things; first, it increases the accuracy of the projectile by eliminating the random drift due to the Magnus effect, and second, it allows a longer, heavier bullet to be fired from the same caliber barrel, increasing range and power (see external ballistics). In the eighteenth century, the standard infantry arm was the smoothbore musket; by the nineteenth century, rifled barrels became the norm, increasing the power and range of the infantry weapon significantly.

Modern smoothbores

Some smoothbore firearms are still used. The most common of these is the shotgun. Firing multiple, non-coaxial projectiles out of a rifled barrel would cause a very high spread resulting in 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.

The cannon, in the form of the tank gun, has made the transition from smoothbore to rifled and 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 just 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, and today all main battle tanks except for the British Challenger 2 & Indian Arjun MBT support smoothbores. The Russian navy also conducted experiments with large-caliber smoothbore navy guns, which were halted by budget cuts.

The tank gun evolution has also shown up in small arms, particularly the U.S. Advanced Combat Rifle (ACR) program. The ACR "rifles" used smoothbore barrels to fire single or multiple flechettes (tiny darts) per pull of the trigger, rather than bullets, 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.

Another smoothbore weapon in use today is the 37-mm riot gun. These are used at short ranges against groups of people, where a high degree of accuracy is not required.
Mortar barrels are typically smoothbores, firing fin-stabilized mortar bombs. Since a mortar bomb is dropped down the barrel under the force of gravity, a smooth barrel makes it easier and faster to load.

Legal issues

In the United States, firearms with a rifled barrel and a bore diameter greater than 0.5 inches (12.7 mm) are generally considered to be destructive devices under the National Firearms Act, and therefore heavily regulated. The exception to this is shotguns with rifled barrels for firing shotgun slugs, or as in the case of the Paradox gun.

See also

• Rifling

Category:Artillery by typeCategory:Firearm terminologyCategory:Artillery components
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