re: ***UPDATE NO.3**My new AR is cycling .223 fine but hanging up on 5.56

Maybe. But what he is saying (bolt not going into battery) and what he is describing (FTF with one caliber) are two different things. For that matter, we might all be thinking too far ahead. What do you think of this?

He might have a lot of misloaded out of spec ammo. Probably first thing would be to get hands on another brand of 5.56 and try that. Unfortunately too common these days (with production at such overtime rates) to hear about out of spec factory ammo. I had five rounds FROM ONE BOX the other day with upside down primers.

I just don't see how people can argue headspace when it is chambering .223 but not 5.56. They are dimensionally identical rounds. I guess it could be bolt bounce and a H2 buffer and / or heavier spring would be the correct treatment for that BUT as a diagnostic procedure that is putting the cart before the horse. Simplest first. Adequate lube, ammo, correct assembly (I have heard of guns coming from some of these places incorrectly assembled), correct gassing (properly aligned gas block/no kinks in tube), buffer/spring, check chamber and lugs for burrs/scratches, etc.

Your simplest diagnostic may be to borrow a buddy's AR and swap your upper onto his and his onto your lower. Then shoot some. That will help you rule out several of the issues brought up here in a couple of mags. As someone pointed out several posts back, some pictures would help.