"Caliber" is used to merely used to describe the barrel length as multiples of the bore diameter. A "5-inch 50 caliber" gun has a bore diameter of 5 inches (127 mm) and a barrel of 250 inches (6.35 m). A .30 caliber rifle uses a .30 inch projectile
Units of length
Many different units of length have been used around the world. The main units in modern use are U.S. customary units in the United States and the Metric system elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.
The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:
7.62 mm caliber
The .50 Browning Machine Gun (.50 BMG) or 12.7×99mm NATO is a cartridge developed for the Browning .50 caliber machine gun in the late 1910s. Entering service officially in 1921, the round is based on a greatly scaled-up .30-06 cartridge.]citation needed[ Under STANAG 4383, it is a standard cartridge for NATO forces as well as many non-NATO countries. The cartridge itself has been made in many variants: multiple generations of regular ball, tracer, armor piercing, incendiary, and saboted sub-caliber rounds. The rounds intended for machine guns are linked using metallic links.
The .50 BMG cartridge is also used in long-range target and sniper rifles, as well as other .50 caliber machine guns. The use in single-shot and semi-automatic rifles has resulted in many specialized match-grade rounds not used in .50 caliber machine guns. A McMillan Tac-50 .50 BMG sniper rifle was used by Canadian Army Corporal Rob Furlong of the PPCLI to achieve what was then the longest-range confirmed sniper kill in history, when he shot a Taliban combatant at 2,430 meters (2,657 yards) during the 2002 campaign in the Afghanistan War. This record was surpassed in 2009 in Afghanistan by a British sniper, though using a .338 Lapua Magnum (8.58×70 mm) rifle.
5"/25 caliber gun
7.62 mm caliber is a nominal caliber used for a number of different cartridges. Historically, this class of cartridge was commonly known as .30 caliber, the Imperial unit equivalent, and was most commonly used for indicating a class of full power military main battle rifle (MBR) cartridges. The measurement equals 0.30 inches or 3 decimal lines, written .3" and read as Three-Line.
7.62 mm refers to the internal diameter of the barrel at the lands (the raised helical ridges in rifled gun barrels). The actual bullet caliber is normally .308 in (7.82 mm), although Soviet weapons commonly use a .311 in (7.91 mm) bullet, as do older British (.303 British) and Japanese cartridges.
20 mm caliber
The 5"/25 caliber gun (spoken "five-inch-twenty-five-caliber") entered service as the standard heavy anti-aircraft (AA) gun for United States Washington Naval Treaty cruisers commissioned in the 1920s and 1930s. The goal of the 5"/25 design was to produce a heavy AA gun that was light enough to be rapidly trained manually The gun was also mounted on pre-World War II battleships and aircraft carriers until replaced by the standard dual-purpose 5"/38 caliber gun, which was derived from the 5"/25 and was similar except for the barrel length. United States naval gun terminology indicates the gun fired a projectile 5 inches (127 mm) in diameter, and the barrel was 25 calibers long (that is, for a 5" bore and a barrel length of 25 calibers, 5" x 25 = 125", or about 3.2 meters).
The 20 mm caliber is a specific size of cannon or autocannon ammunition.
There are few weapons (aside from shotguns and large game hunting rifles) which have been built that fire projectiles between .50 caliber (0.50 inch/12.7 mm, roughly 13 mm caliber) and 20 mm caliber, though the 14.5 mm caliber is used by some Soviet machineguns such as the KPV and antitank rifles such as PTRS, PTRD, and NTW-20.
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