Like he said, make sure of the caliber of your guns, they come in many different calibers but 5.56 is by far the most common. Some of those different calibers require dedicated magazines.
There is one exception to Biddler2's comments. A 223 Wylde chamber can SAFELY fire 5.56 NATO rounds. That is a match grade chamber with a longer leade to allow for longer high ballistic coefficient bullets. There is nothing special or different in the strength of the metal used for 5.56 vs 223, they are the same, only the shape of the chamber where the bullet portion resides is different.