I have a Bushmaster AR-15, can I use 5.56?
I have never had a problem firing 5.56 in a .223, only in reloading due to lower case volume in the 5.56. The article warns of lede dimension (I am guessing these are industry standards for MINIMUM dimension). You could chamber a 5.56 and eject it. If you don’t see any barks on the bullet from where it contacted the rifling in the barrel then you should be good to shoot it.
If your rifle is chambered for 5.56 you can safely use .223. if it is chambered for .223 you shouldn’t use 5.56.
It should be stamped right on the barrel, at least mine is.
Easy mnemonic: math
.223 goes into 5.56; but 5.56 does not go into .223
Question
Ammunition, .223 & 5.56mm
Answer
All Bushmaster rifles are chambered to accept 5.56mm ammunition and always have been. The NATO Spec 5.56mm chambers have a longer “leade” or throat than the SAAMI Spec .223 caliber chambers which have about half of the leade or throat of the 5.56mm chambering. While it is safe to fire both 5.56mm and .223 caliber ammunition in our rifles, the 5.56mm ammunition should not be fired in rifles chambered in .223 caliber as they will develop very high pressures.
http://support.bushmaster.com/?title=General_Information/Ammunition,_.223_%26_5.56mm