The length of the barrel (especially for larger guns) is often quoted in calibers. The effective length of the barrel (from breech to muzzle) is divided by the barrel diameter to give a value. As an example, the main guns of the Iowa-class battleships can be referred to as 16"/50 caliber. They are 16 inches in diameter and the barrel is 800 inches long (16 × 50 = 800). This is also sometimes indicated using the prefix L/; so for example, the most common gun for the Panzer V tank is described as a "75 mm L/70," meaning a barrel with an internal bore of 75 mm, and 5,250 mm long (17 ft 2.69 in).
The bore to barrel length ratio is called caliber in naval gunnery, but is called length in army artillery. Before World War II, the US Navy used 5"/51 caliber (5"/L51) as surface-to-surface guns and 5"/25 caliber (5"/L25) as surface to air guns. By the end of World War II, the dual purpose 5"/38 caliber (5"/L38) was standard naval armament against surface and air targets. All three had a bore diameter of 5 inches (not 5.51 or 5.25 or 5.38 as often misread).
Early gun barrels were short and thick, typically no more than 26 calibers, as the gunpowder propellant they used burned very quickly and violently, and hence its acceleration time was short. Slower-burning "brown powder" formulations of gunpowder allowed gun barrel length to increase slightly in the 1880s but enormous quantities of brown powder were required. New slower-burning "smokeless powder" propellants available from the 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 gasses when burned. Muzzle velocity became limited only by the length of barrel that was feasible, both in terms of the construction methods of the day and in terms of any practical constraints imposed by the gun's manner of use.
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. In internal ballistics terms, if the base of a projectile is thought of as a piston propelled by the expanding gas, then as barrel length increases the volume swept by the piston also increases, and hence the amount of energy that can be extracted from the gas's burning increases. A longer barrel allows more propellant to be used: the propellant is all burned fairly early in the projectile's journey along the barrel, but the projectile continues accelerating as long as the gases generated as a result of the burning continue to exert pressure on its base. This pressure is reduced by the increasing barrel volume the gas has to fill, and in order to achieve maximum muzzle velocity with the shortest barrel length, the projectile should exit the barrel as the gas pressure reduces to a small fraction of the maximum.
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. In World War I 45-caliber naval gun barrels were typical, in World War II 50- to 55-caliber barrels were common, with Germany already manufacturing tank guns of 70 calibers by 1943. Today 60- to 70-caliber barrels are not uncommon, but the latest technology has allowed shorter barrels of 55 calibers to attain muzzle velocities of 5,700 feet per second (Script error m/s), as with the Rheinmetall 120 mm tank gun.
However, as many such guns fire projectiles which are much smaller than the gun bore, using discarding sabots so the relationship of projectile size to barrel length is not as straightforward as with older ordnance.
- ↑ difference in British English and American English spelling
- ↑ Canadian Army. B-GL-306-006/FP-001, 1992-06-01