.223 Wylde chamber

In 1957, during research into the development of a military .22 caliber rifle, the .222 Remington Special was created by a joint effort of Fairchild Industries, Remington Arms, and U.S. Continental Army Command.

[1] In 1972, Fabrique Nationale (FN) created a new type of service ammunition for NATO.

5.56×45mm NATO was based on the .223 Remington cartridge being used by the U.S. Army but had greater range and effectiveness.

The chamber is made with the external dimensions and leade angle found in the military 5.56×45mm NATO cartridge and the 0.2240 in (5.69 mm) freebore diameter found in the civilian SAAMI.

Freebore is a short and smooth section of the barrel that is located after the case mouth, located before the start of the rifling "grooves and lands", also often referred to as leade.