A hoop gun is a gun

Gun

A gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol,...

 production

Manufacturing

Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...

 technique that uses multiple layers of tubes to form a built-up gun

Built-up gun

The term built-up gun describes a construction technique for artillery barrels. An inner tube of metal which stretches most within its elastic limit enlarges under the pressure of confined powder gases to transmit stress to outer cylinders under tension...

. The innermost tube has one or more extra tubes wrapped around the main tube. These outer tubes are preheated before they are slid into position. As the outer tubes cool

Cooling

Cooling is the transfer of thermal energy via thermal radiation, heat conduction or convection. It may also refer to:-Techniques:* Air conditioning* Air cooling* Computer cooling* Cryogenics* Conduction * Infrared solar cells* Laser cooling...

 they naturally contract

Contraction

Contraction may refer to:In physiology:* Muscle contraction, one that occurs when a muscle fiber lengthens or shortens** Uterine contraction, contraction of the uterus, such as during childbirth* Contraction, a stage in wound healing...

. This pre-stresses the main tube so it can withstand greater internal pressures.