A pivot gun was a type of cannon

Cannon

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...

 mounted on a fixed central emplacement which permitted it to be moved through a wide horizontal arc. They were a common weapon aboard ships and in land fortifications for several centuries but became obsolete after the invention of gun turret

Gun turret

A gun turret is a weapon mount that protects the crew or mechanism of a projectile-firing weapon and at the same time lets the weapon be aimed and fired in many directions.The turret is also a rotating weapon platform...

s. By mounting a cannon on a pivot, a much wider arc of fire could be obtained than was possible with conventional carriage-mounted cannons. Unlike the latter, however, pivot guns were fixed in one place and could not easily be moved outside of their horizontal arc; they could thus only really be used in fixed positions such as in a fort or on a battleship.

There was no standard size of pivot gun, though they tended to be fairly substantial weapons. Like other cannons, they were muzzleloader

Muzzleloader

A muzzleloader is any firearm into which the projectile and usually the propellant charge is loaded from the muzzle of the gun . This is distinct from the more popular modern designs of breech-loading firearms...

s and could fire either shells

Shell (projectile)

A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...

 or grapeshot

Grapeshot

In artillery, a grapeshot is a type of shot that is not a one solid element, but a mass of small metal balls or slugs packed tightly into a canvas bag. It was used both in land and naval warfare. When assembled, the balls resembled a cluster of grapes, hence the name...

 (or other types of shot). Their calibers ranged from a few inches to the giant 11-inch Dahlgren guns

John A. Dahlgren

John Adolphus Bernard Dahlgren was a United States Navy leader. He headed the Union Navy's ordnance department during the American Civil War and designed several different kinds of guns and cannons that were considered part of the reason the Union won the war...

 used by the United States Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 in the mid-19th century.

Pivot guns had a major disadvantage in warfare: they were very difficult to protect in battle and were necessarily very exposed, as they lay close to the surface of a ship's deck and required an open field of view. In the late 19th century they were replaced by "disappearing gun

Disappearing gun

A disappearing gun is a type of heavy artillery for which the gun carriage enabled the gun to rotate backwards and down into a pit protected by a wall or a bunker after it was fired...

s" and ultimately by turrets, which enabled a broad arc of fire while providing the gunners with all-round protection from incoming fire.

Pivot guns should not be confused with swivel gun

Swivel gun

The term swivel gun usually refers to a small cannon, mounted on a swiveling stand or fork which allows a very wide arc of movement. Another type of firearm referred to as a swivel gun was an early flintlock combination gun with two barrels that rotated along their axes to allow the shooter to...

s, a much smaller type of ordnance.