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
Thanks, I just looked at my Bushmaster and it does have the imprint; .223 and 5.56. This thread has been great as I had no idea about the difference in rounds.