A .223 Wylde chamber is used on .223 caliber rifle barrels to allow them to safely fire either .223 Remington or 5.56x45mm NATO ammunition.
.223 Wylde chamber Wikipedia
In 1957 during research into development of a military .22 caliber rifle the Remington .222 Special was created. Due to there being several .222 caliber cartridges under development, the .222 Special was renamed to .223 Remington. The cartridge became the standard for the US Army in 1962 as the M193 Cartridge.
In 1972 Fabrique Nationale (FN) created a new class of ammunition for NATO. It was based on the .223 Remington cartridge which was being used by the US Army that had greater range and effectiveness. The first iteration of this ammunition was type designated SS109.
Although a rifle barrel that is chambered in .223 Rem can fire 5.56mm it is not advised on account of 5.56mm having higher pressure. This is due to the longer and heavier projectile used in SS109 (M855) ammunition leading to diminished case capacity and a higher loading density which increases chamber pressure with the same powder charge.
Due to differences in the chamber dimensions between the .223 Rem and 5.56 x 45mm NATO (SS109, M-855), the two cannot be used interchangeably, however .223 Remington can safely be fired from firearms chambered for 5.56 mm NATO. As most rifle makers moved to support the 5.56mm NATO specification the reduced accuracy was considered a problem.
Bill Wylde of Greenup, Illinois compared the two cartridges and changed the chamber of the rifle's barrel to a specification which is called 223 Wylde chamber or 22 Wylde chamber. The chamber is made with the external dimensions and lead angle as found in the military 5.56×45mm NATO cartridge and the 0.2240 inch free bore diameter as found in the civilian SAAMI .223 Remington cartridge. Rifles with a .223 Wylde chamber will typically accept both .223 Remington and 5.56×45mm NATO ammunition. Note that while the .223 Remington and 5.56×45mm NATO chambers have slightly different dimension, the cartridges themselves are identical in dimension. The chamber dimension differences are often confused with the cartridge dimensions and so it is often erroneously thought that the cartridges have different dimensions. The cartridges are loaded to different pressure levels (with the 5.56 being greater), however. (The NATO dimensions offset the higher pressure with a longer throat - often confusing some to think the cartridge itself is longer). This chamber allows the use of both pressure levels safely while also increasing accuracy potential across the range of potential pressures.
Wylde's hybrid chamber was designed to exploit the accuracy advantages of the .223 Remington chambering without problems concerning over pressure or compromising the functional reliability of semi-auto firearms like the AR-15 family when using 5.56×45mm NATO military ammunition. Coincidentally, it shoots the relatively long and heavy 80-grain (5.18 g) bullets commonly used in the Sport Rifle Competition very well and is one of the preferred chambers for that use. The Wylde chamber is used by rifle manufacturers who sell "National Match" configuration AR-15 rifles, barrels, and upper receivers.