You can fire .223 in a gun chambered for 5.56, but the 5.56 chamber has a longer lead, and because of this, accuracy may suffer and velocity will be slightly less. The more important issue is the 5.56 loaded to military specs has higher velocity and chamber pressure than the .223. Consequently, firing 5.56 ammo in a .223 chamber with a shorter lead can dramatically increase chamber pressure. In some cases this can result in primer pocket gas leaks, blown cartridge case heads and functioning issues. The 5.56 military cartridge fired in a .223 Rem chamber is considered by SAAMI to be an unsafe ammunition combination. This is why most high quality, accuracy-built AR-15 models offered with 5.56 chambers instead of .223 chambers, so people won't be loading .223 chambers with 5.56 ammo - the other way is ok, it's a safety issue, not an accuracy one. The RRA Wylde chamber was designed as a Match chambering and is relieved slightly to help in extraction and has a shorter lead to improve accuracy - but it will accept both .223 and 5.56.will a barrel designated for the 5.56 nato fire the .223 round safely? is there really that much of a difference in the two rounds? i know the .223 wylde chamber will handle both rounds, but how do barrels work. i've already started assembling the lower, and im looking ahead at parts i will need next. dont want to hurt myself when i go to the range with my first build.