It's my understanding that it's all about the lead/throat . A 5.56 chamber/barrel has a longer lead/throat then the 223 . This allows room for the longer heavier NATO rounds like tracers , green tip and others .Remember we are talking about NATO ammo made in many countries . There are some 5.56 NATO rounds that if put in to a 223 chamber the bullet would be pushed in to the lans of the rifling . When a gun is fired with the bullet slightly pushed into the rifling , this can create a pressure spike well above recommended levels . It's my understanding just a little lead ( fractions of an inch ) can make a big difference of pressure inside the chamber .
Again thats my understanding and it may not be fact . Any corrections are welcome
Last edited by Metal god; November 28, 2012 at 06:43 AM.