Because the military switched to metric designations sometime in the 1950s, this little . 22-cal. cartridge was later called the 5.56x45 mm NATO (commonly referred to as “5.56x45 mm”). The 5.56x45 mm surfaced in 1957 as an experimental cartridge in the AR-15 rifle.

The 5.56 NATO is one of the best infantry fighting calibers in the world. It's fought for decades and done an excellent job at its role as a general infantry cartridge. Sure, it's not perfect, but the cartridge has proven to be a reliable warfighting cartridge.

The biggest difference in the construction of 5.56 versus . 223 is the amount of pressure produced when firing. In standard production rounds, 5.56 NATO ammunition is packed with slightly more powder, creating a slightly faster round. 5.56 ammo also utilizes a heavier bullet than most off-the-shelf .

223 and 556 ammunition is among the most available and cost-effective ammo options, used for target shooting, competition, personal defense, varmint, or even small game hunting. You'll find all kinds of semi-auto rifles, bolt-action rifles, carbines, and even pistols chambered in both rounds.

The 5.56×45mm NATO round, pronounced "five-five-six", is an intermediate cartridge and the main bullet type used by guns made for NATO militaries. It was first used in the Vietnam War as the main bullet used by the US military.

There is no difference between 5.56x45mm and 5.56 NATO. They are the exact same cartridge, just using a slightly different name. The former is simply a dimensional description, while the latter, 5.56 NATO, is the official nomenclature.