Smooth-bore

A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars. The majority of shotguns are smoothbores and the term can be synonymous.

Early firearms had smooth barrels that fired projectiles without significant spin. To minimize inaccuracy-inducing tumbling during flight their projectiles required stable shape, such as a sphere. However, the Magnus effect causes even spheres rotating randomly during flight to curve when spinning on any axis not parallel to the direction of travel.

Rifling a barrel with spiral grooves or polygonal rifling imparts a stabilizing gyroscopic spin to a projectile that prevents tumbling in flight. Not only does this more than counter Magnus-induced drift, but it allows a longer, heavier round to be fired from the same caliber barrel, increasing both range and power.

In the eighteenth century, the standard infantry arm was the smoothbore musket; 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 the Minié ball increased their rate of fire to match that of smoothbores.

Artillery weapons were smoothbore until the middle 19th century, and smoothbores continued in limited use until the 1890s. Early rifled artillery pieces were patented by Joseph Whitworth and William Armstrong in the United Kingdom in 1855. In the United States, rifled small arms and artillery were gradually adopted during the American Civil War. However, heavy coast defense Rodman smoothbores persisted in the US until circa 1900 due to the tendency of the Civil War's heavy Parrott rifles to burst and lack of funding for replacement weapons.

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