.223-tighter chamber, short throat
.223 Wylde-tighter than 5.56 chamber and throat, but looser than .223 Rem
5.56-looser chamber and throat, but built for higher pressures

I've shot 5.56 in my .223 chambered ARs without issue. If it's made by one of the more reputable companies, it shouldn't have a problem with the ammo. Some choose not to shoot it though.

Steel ammo kills extractors. Only stuff I've ever seen actually break a Glock extractor. Popped a piece right off the bottom of the claw. I reload, no steel cases for me.