What is "built-up gun"

A built-up gun is artillery with a specially reinforced barrel. An inner tube of metal stretches within its elastic limit under the pressure of confined powder gases to transmit stress to outer cylinders that are under tension. Concentric metal cylinders or wire windings are assembled to minimize the weight required to resist the pressure of powder gases pushing a projectile out of the barrel. Built-up construction was the norm for guns mounted aboard 20th century Dreadnoughts and contemporary railway guns, coastal artillery, and siege guns through World War II.