.223 Wylde chamber

A .223 Wylde chamber is a wildcat rifle chamber designed to accurize the 5.56×45mm NATO chamber for NRA High Power competition.

Background
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. Based on the .222 Remington, and with several other .222 caliber cartridges under development for civilian rifles at the same time, the .222 Remington Special was renamed .223 Remington. In 1963 it was adopted as the standard intermediate cartridge for the United States Army, as the M193 Cartridge.

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 first iteration of this ammunition was designated SS109 in NATO countries, and later adopted in the U.S. as the M855.

Chamber dimensions
Bill Wylde of Greenup, Illinois, compared the two cartridges and changed the chamber of the rifle's barrel to a specification he called the .223 Wylde chamber. 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 freebore diameter found in the civilian SAAMI.

The .223 Wylde chamber is used by rifle manufacturers who sell "National Match" configuration AR-15 rifles, barrels and upper receivers.

Comparison
The major dimensional difference between the chambers that fire the .223 Remington and the 5.56×45mm NATO is the longer and larger-diameter "freebore" in the 5.56 chamber (0.0566 in vs 0.0250 in length, 0.2265 in vs 0.2240 in diameter). 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".