If it's stamped on the side of the reciever then it's ok to use. If it's stamped 5.56mm you can use 5.56 or .223. If it's stamped .223 you should only use that round in it.
Some AR's are manufactured only for the .223 round and it can be dangerous to use the hotter 5.56mm round in them.
My AR, however, says 5.56mm (.223) which allows me to use either.
Century Arms AR-15A2, 5.56
1946 Mosin-Nagant M44, 7.62x54R
1978 YUGO SKS, 7.62x39
NEF Protector Pump, 12 gauge
Savage Model 64, .22LR
NEF Pardner Tracker II, 12 gauge
Rock Island Armory 1911A1, .45 ACP
Remington 870 'Express Magnum' 12 gauge
Bersa Thunder .380