The 5.56x45 cartridge was developed after extensive research into small caliber, high-velocity (SCHV) projectiles in the late 1950's and early 1960's and became one of the most popular centerfire cartridges in the world. Law enforcement agencies and militaries around the world use the cartridge, as do an increasing number of civilians in the United States. The .223 Remington is a sister cartridge that was inspired by the 5.56x45, and it can be fired from rifles chambered for the 5.56 (but for safety's sake, not the other way around).
The 5.56x45 was first loaded with full metal jacket bullets weighing 55 grains that exited the barrel of the new AR-15 rifle at more than 3,200 feet per second. The AR-15 eventually became the M16. Since then, many different projectiles have capped this cartridge, including the M855 - a steel penetrator tip, and the Mk318 - a barrier-blind. Both of these bullets weigh 62 grains. For precision marksmanship, the 5.56 cartridge uses a Sierra MatchKing 77 grain Boat Tail Hollow Point, or the Mk262 Open Tip Match bullet.
Popular worldwide, many manufacturers of firearms have produced weapons chambered for the 5.56x45 cartridge. Armalite, Bushmaster, Colt and others have manufactured semi-automatic rifles and fully automatic rifles chambered in 5.56x45.