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Asmoothbore weapon is one that has abarrel withoutrifling. Smoothbores range from handheldfirearms to powerfultank guns and large artillerymortars. Some examples of smoothbore weapons are muskets, blunderbusses, and flintlock pistols. The opposite of smoothbore isrifling.
Early firearms had smoothly bored barrels that firedprojectiles without significant spin.[1] To minimize inaccuracy-inducing tumbling during flight, their projectiles required an aerodynamically uniform shape, such as asphere. However, surface imperfections on the projectile and/or the barrel will cause even a sphere to rotate randomly during flight, and theMagnus effect will curve it off the intended trajectory when spinning on any axis not parallel to the direction of travel.[2]
Rifling the bore surface with spiral grooves orpolygonal valleys imparts a stabilizinggyroscopic spin to a projectile that prevents tumbling in flight. Not only does this more than counter Magnus-induced drift, but it allows a longer, more streamlined round with greatersectional density to be fired from the samecaliber barrel, improving theaccuracy,effective range andhitting power.
In the eighteenth century, the standard infantry arm was the smoothboremusket; although rifled muskets were introduced in the early 18th century and had more power and range, they did not become the norm until the middle of the 19th century, when theMinié ball increased their rate of fire to match that of smoothbores.[3]
Artillery weapons were smoothbore until the mid-19th century, and smoothbores continued in limited use until the late 19th century. Early rifled artillery pieces were patented byJoseph Whitworth andWilliam Armstrong in the United Kingdom in 1855. In the United States, rifled small arms and artillery were gradually adopted during theAmerican Civil War. However, heavy coast defenseRodman smoothbores persisted in the US until 1900 due to the tendency of the Civil War's heavyParrott rifles to burst and lack of funding for replacement weapons.
Some smoothbore firearms are still used.
Ashotgun fires multiple, roundshot; firing out of a rifled barrel would impartcentrifugal forces that result in a doughnut-shaped pattern of shot (with a high projectiledensity on the periphery, and a low projectile density in the interior). While this may be acceptable at close ranges (some spreaderchokes 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]
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Another smoothbore weapon in use today is the 37-mmriot gun, which firesless-lethal munitions likerubber bullets andteargas at short range at crowds, where a high degree of accuracy is not required.[5]
TheSteyr IWS 2000 anti-tank rifle is smoothbore. This can help accelerate projectiles and increase ballistic effectiveness. The projectile is a 15.2 mmfin-stabilized discarding-sabot type with armor-piercing capability which the IWS 2000 was specifically designed to fire. It contains a dart-shaped penetrator of eithertungsten carbide ordepleted 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 firingcannonballs to rifled firingshells in the mid-19th century. However, to reliably penetrate the thick armor of modern armored vehicles many moderntank guns have moved back to smoothbore. These fire a very long, thinkinetic-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-stabilizeddarts. Not only does this reduce the time and expense of producing rifled barrels, it also reduces the need for replacement due to barrel wear.[citation needed]
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 multipleflechettes (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 poorterminal ballistics.[citation needed]
Mortar barrels are typicallymuzzle-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.
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