Remember the old advertising slogan, "When E.F. Hutton talks, people listen"? In the firearm industry, when SAAMI talks, we listen. The Sporting Arms & Manufacturers' Institute sets the standards for ammunition and firearms manufactured in the United States (details at SAAMI.org). The folks at SAAMI got in touch with the Smyth Busters after we released our video "Is It Safe To Shoot 5.56 in a .223 AR-15?" The ammunition you load in any gun should match the markings on the outside of the gun. With an AR-15, use the ammunition type marked on the BARREL. AR-15 receivers are sometimes marked vaguely with something like "Cal. MULTI". If your AR-15 has a .223 Remington chamber cut tighter than SAAMI specs, you should NOT fire 5.56 NATO ammo in it.

As a military round, the 5.56x45mm NATO is designed to work in a variety of firearms, and there are multiple 5.56 loads, all of which generate HIGHER pressures than civilian .223 Remington loads. Military ammo is also pressure tested using a different method from civilian ammunition. The resulting pressure statistics are not comparable - an "apples to steak knives" sort of comparison!

Higher-pressure ammunition will wear out your firearm more rapidly because higher bolt thrust puts more strain on the gun's moving parts. SAMMI defines a pierced primer, blown primer, a torn off cartridge rim, and the like as an ammunition failure. You should stop using that ammo in that gun if any of these things happen.