Some ARs shoot .223 ammunition. Others fire only 5.56mm. Still others chamber both. You can fire a .223 from a 5.56 gun but you’re advised not to do the opposite. Most shooters understand that. The reason why may be lost on some though. What exactly is the difference?
AGG.com featured a video article on the differences: .223 vs. 5.56. Please look at their article and watch the following video.
.223 Vs. 5.56
That’s why you can run a .223 through a 5.56 gun but not the other way around. If in doubt (and in the market) buy a gun designed for both.