Caliber (artillery)
Steel artillery projectiles may have a forward bourrelet section machined to a diameter slightly smaller than the original land-to-land dimension of the barrel and a copper driving band somewhat larger than the groove-to-groove diameter to effectively seal the bore as it becomes enlarged by erosion during prolonged firing.
United States Navy guns typically used rifling depth between one-half and one percent of caliber.
By the end of World War II, the dual purpose 5-inch/38-caliber gun (5" L/38) was standard naval armament against surface and air targets.
[3][4][5][6] For naval rifles, the initial change was to actual bore, thus facilitating the manufacture of standard projectiles.
This explains the differences in both penetration and long range performance of various naval rifles over the years.
Later improvements to the design, lengthening the rifle itself and also altering the breech, allowed a 1,400 lb (640 kg) projectile and, overall, a greater barrel life.
New slower-burning "smokeless powder" propellants available from the mid-1880s onwards, such as Poudre B, cordite and nitrocellulose allowed a gentler prolonged acceleration, hence gun barrels were made progressively longer and thinner.
The new formulations were far more powerful propellants than gunpowder and far less was needed by weight as they transformed almost entirely to gases when burned.
The practical effect of long barrels for modern guns is that the projectile spends more time in the barrel before it exits, and hence more time is available for expanding gas from the controlled burning of the propellant charge to smoothly accelerate the projectile, bringing about a higher velocity without placing undue strain on the gun.
A longer barrel allows more propellant to be used: the propellant is all burned fairly early in the projectile's journey along the barrel, except in the very common instance where combustion is still occurring as the projectile leaves the muzzle and a visible muzzle "flash" is produced.
Technological improvements had made it possible to introduce into use long gun barrels that are strong enough to withstand the forces involved in accelerating the shell to a high velocity, while remaining light enough to be reasonably mobile, rigid enough to maintain accuracy, and having a bore able to withstand many firings before needing refurbishment.
