You never came across this basic information before in the military?
You should always buy 5.56x45mm for general use. Then you can shoot both 5.56x45mm and .223Rem. The .223Rem rifle is used primarily as a match weapon and cannot handle the higher pressure of the 5.56x45mm round.
There are a million threads on FTF answering this question. Perhaps BING?
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face