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 . Concentric metal cylinders and / 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 Dreadnought s and contemporary railway gun s , coastal artillery , and siege guns through World War II . Background . Velocity and range of artillery vary directly with pressure of gunpowder or smokeless powder gasses pushing the shell out of a gun barrel ; but a gun will be deformed or explode if chamber pressures 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 . Nomenclature . 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 . Assembly Procedure . After the tube , jacket , and hoops have been machined to appropriate dimensions , the jacket is carefully heated to approximately 400 degrees Celsius ( 800 degrees Fahrenheit ) in a vertical air furnace so thermal expansion allows the cool tube to be lowered into place . When the jacket is in position , it is cooled to form a tensioned shrink fit over the tube . Then the next hoop ( either B2 or C1 ) is similarly heated so the assembled A tube and B1 jacket can be lowered into position for a successive shrink fit . The assembled unit may be machined prior to fitting a new hoop . The process continues as remaining tubes are heated sequentially and cooled onto the built - up unit until all elements have been assembled . When tensioned wire winding is used in place of a hoop course , the wire is typically covered by an outer tensioned cylinder also called a " jacket " . Liners . Burning powder gasses melt part of the bore each time a gun is fired . This melted metal is oxidized or blown out of the muzzle until the barrel is eroded to the extent shell dispersion becomes unacceptable . After firing several hundred shells , a gun may be reconditioned by boring out the interior and inserting a new liner as the interior cylinder . Exterior cylinders are heated as a unit to approximately 200 degrees Celsius ( 400 degrees Fahrenheit ) to allow insertion of a new liner and the liner is bored and rifled after installation . A new liner may be bored for a different projectile diameter than used in the original gun . Liners may be either cylindrical or conical . Conical liners are tapered toward the muzzle for ease of removal from the breech end while limiting forward creep during firing . Conical liners may be removed by water cooling the liner after re - heating the barrel , but cylindrical liners must be bored out . Monoblock Guns . With the obsolescence of very large guns following World War II , metallurgical advances encouraged use of monoblock ( one - piece ) construction for postwar guns of medium caliber . In a procedure called autofrettage , a bored monoblock tube is filled with hydraulic fluid at pressures higher than the finished gun will experience during firing . Upon release of hydraulic pressure , the internal diameter of the monoblock tube will have been increased by approximately six percent ; but the outer portion of the finished monoblock rebounds to approximate its original diameter and exerts compressive forces on the inner portion similar to the separate cylinders of a built - up gun . See also . Hoop gun