5.56mm NATO
Coming out of the barrel of the M16 is the 5.56mm NATO. I bring up NATO here since it has not only been adopted by the US, but nearly all of our allies as well. This was the entire point of the round’s development: if NATO ever did go head to head with the Soviet Union, the leadership of NATO wanted the countries to share as many supply lines as possible, and sharing a bullet in common for rifles and machine guns was a major part of that, and it still is today. Many countries use the M16, and many of those who don’t use weapons that take AR magazines.