I just picked up some 1X fired brass at the range from some guys shooting next to me stamped M193 and the other side 80 which I presume is the year. They were shooting AR-15's. I believe it is 5.56mm but it does not say .5.56 or .223 on the case head. I have never loaded anything with this head stamp. Is it Nato spec? It's probably common knowledge but I have been shooting 30 cal. for many years. If one of you knowledgeable guy's would clue me in I would be thankfull. Thanks in advance.