Quote Originally Posted by Tacgunner View Post
One at a time, dropped into chamber by hand without the bolt: factory brass .223, mil-spec brass cased 5.56mm, your reloads.

Do they all drop in and drop out without problem? If not, which ones don't fit? If all stick, then chamber issue. Clean and recheck. If still problem, do chamber cast and measure.

This isolates an ammo problem, or chamber issue.

Checking headspace and bolt/bolt carrier are next.
My thoughts exactly, I'll just use Tacgunner's post so I don't need to retype it.

Occasionally we will have an MG like this and if a really good cleaning doesn't fix it, we check all the springs, extractor, ejector, etc, then if still not working, we will put a different lower on it to eliminate buffer issues, then if still not working, we will put a chamber brush on an drill and polish up the chamber for about 30 seconds, test again, and repeat couple of times before we start ripping about the gas system.