I HATE this question. I sell guns for a living now that I am retired from full time work. I get so tired of answering this question. The techno geeks say it is unsafe and that the rounds are different, Yes and Yes BUT it is like saying Can I blow a head gasket at 14 lbs boost. Yes, but highly unlikely. There are very minor dimensional differnences in the 2 rounds but the issue is that 5.56 is military and they make their cases thicker to hold up to full auto use. They also make the chamber a tad looser so it will chamber and continue to shoot when it gets real hot (and swells) after sustained fire. That means IF you shoot a 5.56 in a 223 chamber, the case cannot expand quite as much and WILL create more chamber pressure. But the issue is these damn guns are built and pressure tested at close to 3 times the normal pressure so that little bit of extra pressure doesn't mean a damn. I shot comprtition with M1A's and AR's for MANY years and used 7.62 AND .308 in the M1A and .223 and 5.56 in the AR with only one problem and that wasn't a danger problem. My M1A had a tight match chamber and I was in a match called Rattle Battle at the Highpower Nationals in Camp Perry. We were issued US Lake City ball 7.62 and the match was several hundred rounds as quick as you wanted to shoot them starting at 600 yds, then 500 then 400 etc. About half way through my tight chamber expanded to where the GI rounds would NOT chamber in the gun. Had nothing to do with dangerous high pressure. I know I will catch crap fpr this post but real life is real life. YES they are different and will create more chamber pressure but I have never seen anyone able to document a case where it blew up a gun. Another issue is that some of the guns these days are marked 5.56 and some .223 and some BOTH. Problem is that some manufacturers cut their chambers to the slightly bigger 5.56 but mark the gun .223. ALSO remember the receiver markings have nothing to do with the barrel where the chamber is. We had a Colt .223 AR but it came from the factory chambered for 7.62 39 (AK stuff) and some manufacturers mark their barrels and some don't. So if you have an off brand gun, who made the barrel on that Spikes AR lower and what did they cut the chamber to??
So bottom line is Yes they are different but........