This is the approximate size of a .45 caliber round, the conversion being .443 inches.
5.56mm x 45mm (5.56 is the diameter of the buller, 45mm is the length of the brass casing). NATO implies that this is THE 5.56mm round used by NATO forces. Similar to 7.62mm x 51mm NATO is the standard .30 caliber round use by NATO forces.
it fires a 5.56 by 51 milimeter standard NATO round. no... the m-16 and m4 are 5.56 x 45 NATO. 5.56 x 51 doesnt exist. its a 7.62 x 51 NATO round that an m-14 fires.
5.56x45mm NATO
7.62 NATO
5.56x45 NATO
The 7.62 NATO round is bigger that the 5.56 NATO round
7.62x51 NATO
The .308 Winchester is equivalent to the 7.62x51mm NATO cartridge, as they share the same dimensions and ballistic characteristics. While the .308 is a commercial round, the 7.62 NATO is its military counterpart. Both cartridges can be fired from the same rifles designed for either caliber, though it's important to check specific firearm compatibility.
The manufacturer is spelled Barrett (Barrett Firearms Manufacturing), builder of the M82 and XM500 .50 caliber sniper rifles that fire the 12.7x99mm NATO round (Browning machine gun round).
7.62 mm NATO AmmoThe first standard NATO cartridge, the 7.62 x 51mm NATO, was developed by the United States as a successor to the .30 Caliber M2 round (30-06), which had served as the standard U. S. rifle cartridge since 1906.The standard .30 Caliber M2 cartridge propelled a 150 grain projectile at a muzzle velocity of 2,800 fps (848 mps). That's 848 meters per second not miles.
It's more a matter of where it's manufactured and by whom than the caliber. Several non-NATO nations also manufacture NATO standard ammo. For example, Russia manufactures the 5.56x45 and 7.62x51 to go along with export versions of NATO caliber firearms they manufacture, such as the AK-101. Their steel cased rounds are much less expensive than, say, American made M855 ammo.
Non-American nations typically use the metric system to denote the size of their ammunition, as does the US military. The 7.62mm round is equivalent to the .30 caliber round, which is 30-hundredths of an inch.