Just noticed on my Steyr USR, the barrel is stamped .223 rem. I thought these guns were made for .223 or 5.56mm. I couldn't find anything on a seach. Just bought 1000 rounds of Q3131 2005. I hear that this is excellent ammo, [from AR15.com] but is it safe to shoot in this gun? I know that a gun that is stamped .223 shouldn't use 5.56. Am I wrong here?