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.223 Wylde --

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 externally slightly larger 5.56×45mm NATO ammunition.[1]

The .223 Wylde hybrid chamber was designed by Bill Wylde of Greenup, IL to exploit the accuracy advantages of the .223 Remington chambering without pressure problems or compromising the functional reliability of (semi) automatic weapons like the AR-15 family when using 5.56×45mm NATO military ammunition.[2] Coincidentally, it shoots the relatively long and heavy 80-grain (5.18 g) bullets commonly used in the sport of Highpower 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.