Caliber ( artillery ) In artillery , caliber or calibre is the internal diameter of a gun barrel , or by extension a relative measure of the length . Rifled barrels . Rifled barrels introduce ambiguity to measurement of caliber . A rifled bore consists of alternating grooves and lands . The distance across the bore from groove to groove is greater than the distance from land to land . Projectiles fired from rifled barrels must be of the full groove to groove diameter to be effectively rotated by the rifling , but the caliber has sometimes been specified as the land to land diameter before rifling grooves were cut . The depth of rifling grooves ( and the consequent ambiguity ) increases in larger calibers . 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 . Projectile bourrelet diameter specification was less than land to land diameter with a minus manufacturing tolerance so average clearance was about . Driving band diameter was groove to groove diameter plus . Barrel length . 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 dimensionless quantity . As an example , the main guns of the s 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 ) . Naval rifles , although constructed and manufactured in roughly the same manners as land based artillery , were built to much more stringent and studious standards than land based weapons , and for good reason . At sea , a weapon had to perform , without fail . There was no ready replacement , nor one that could be readily supplied . Over time , the terms of pound ( weight of shell ) and bore ( the actual bore of the weapon ) became confused and blurred . Eventually , when the technology existed , the bore ( in inches or millimetres ) came to be the standard measure . For naval rifles , the initial change was to actual bore , thus facilitating the manufacture of standard projectiles . They then began to measure the effective length ( and therefore range ) of the weapon in calibers . These were ( and are ) a measure of the standardized bore of the barrel versus the rifled bore of the barrel . In other words , a 12 / 45 is 12 " X45 = the length of the rifled bore of that gun in inches . This explains the differences in both peneration and long range performance of various naval rifles over the years . In addition to the possible improvements in overall performance ( i . e . muzzle velocity and striking force ) , the increase in barrel length also allowed , in some circumstances , an increase in projectile size as well . For example , the American 14 / 45 , as introduced in the s , fired a 1250 lb . projectile . Later improvements to the design , lengthening the rifle itself and also altering the breech , allowed a 1400 lb . projectile and , overall , a greater barrel life . Again we see this pattern with the US 16 " guns . The initial design was 45 calibers in length and fired a 2200 lb . shell . The later re - design to 50 calibre not only allowed a higher velocity but also a heavier 2700 lb . shell , which ultimately came to be accepted as the greatest naval shell ever deployed in combat . 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 , 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 sabot s so the relationship of projectile size to barrel length is not as straightforward as with older ordnance .