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.

Velocity and range of artillery vary directly with pressure of gunpowder or smokeless powder gasses pushing the shell out of a gun barrel. A gun will deform (or explode) if chamber pressure strain a gun barrel beyond the elastic limit of the metal from which the barrel is made. Thickness of homogeneous cast metal gun barrels reached a useful limit at approximately one-half caliber. Additional thickness provided little practical benefit, since higher pressures generated cracks from the bore before the outer portion of the cylinder could respond, and those cracks would extend outward during subsequent firings.

Claverino's 1876 treatise on the "Resistance of Hollow Cylinders" was published in Giornale d'Artigliera. The concept was to give exterior portions of the gun initial tension, gradually decreasing toward the interior, while giving interior parts a normal state of compression by the outer cylinders and wire windings. Theoretical maximum performance would be achieved if the inner cylinder forming the rifled bore were compressed to its elastic limit by surrounding elements while at rest before firing, and expanded to its elastic limit by internal gas pressure during firing.

The innermost cylinder forming the chamber and rifled bore is called a tube or, with certain construction techniques, a liner. A second layer cylinder called the jacket extends rearward past the chamber to house the breechblock. The jacket usually extends forward through the areas of highest pressure, through the recoil slide, and may extend all the way to the muzzle. The forward part of the barrel may be tapered toward the muzzle because less strength is required for reduced pressures as the projectile approaches it. This tapered portion of barrel is called the chase. Very large guns sometimes use shorter outer cylinders called hoops when manufacturing limitations make full length jackets impractical. Hoops forward of the slide are called chase hoops. The jacket or forward chase hoop may be flared outward in the form of a bell at the muzzle for extra strength to reduce splitting because the metal at that point is not supported on the forward end. As many as four or five layers, or hoop courses, of successively tensioned cylinders have been used. Layers are designated alphabetically as the "A" tube enclosed by the "B" jacket and chase hoops, enclosed by the "C" hoop course, enclosed by the "D" hoop course, etc. Individual hoops within a course are numbered from the breech forward as the B1 jacket, the B2 chase hoop, and then the C1 jacket hoop, the C2 hoop etc. Successive hoop course joints are typically staggered and individual hoop courses use lap joints in preference to butt joints to minimize the weakness of joint locations. Cylinder diameter may be varied by including machined shoulders to prevent forward longitudinal movement of an inner cylinder within an outer cylinder during firing. Shoulder locations are similarly staggered to minimize weakness.

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Wikipedia