Created an account just to share a couple of links. First to an article that is well worth reading all 4 pages, then to a site that offers a ".223/5.56? gage" that is the only difinitive way to determine if you have a .223 chamber or a 5.56 chamber.
http://www.gundigest.com/223-vs-5-56
http://www.m-guns.com/tools.php click the pic of the 5.56 reamer and the .223/5.56? gage to read more.