The .223 Wylde is a proprietary rifle cartridge chamber with the external dimensions and lead angle as found in the military 5.56×45mm NATO cartridge and the 0.2240 inch freebore 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.
The .223 Wylde hybrid chamber was designed by Bill Wylde of Greenup, Illinois, to exploit the accuracy advantages of the .223 Remington chambering without pressure problems or compromising the functional reliability of semi-auto weapons 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 a few rifle manufacturers who sell "National Match" configuration AR-15 rifles, barrels, and upper receivers.