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.
556
45mm
5.56×45mm NATO is a rifle cartridge developed in the United States
555 plus 5 plus 556 is equal to 1,116
563 + 556 = 1019
4589 + 5865588 - 556 = 5869621
456 plus 100 is equal to 556
5 + 26 + 5 + 6 + 8 + 68 + 53 + 556 + 565252 + 52 = 566031
45mm.
556 + 323 = 879 whose digits sum to 24.
45mm = 4.5cm
5.56 x 45mm NATO is the caliber. The weight is 6.2 lbs, and with stock open, is 28.3 inches long. With stock collapsed, 19.7 inches.