Martin,

It may be an optical illusion, but apart from the case being ruptured, it looks swollen out, as if the chamber is grossly wrong (too large - not for .223). Adding to that thought, that's an unusual case rupture UNLESS the chamber is jugged out.

The firing pin indent seems a little large (diameter) for a caliber that uses a small rifle primer, but perhaps all NEF frames use the same firing pin (?) That looks like an indent left on a shotshell primer, FWIW. <shrug> Shouldn't make an difference, other than being overkill. Primer was not pierced from what I can see (no scorch marks radiating from the primer / primer pocket area)

Sounds like the lock-up may be twitchy but no way for me to tell without the rifle in-hand. But you would have noticed for certain if the action was not locked when it fired, even with a .223.

Is the ammo loaded with 55gr or 62gr bullets? What is the twist rate on the rifling (you can check that with a tight patch and a cleaning rod)? These are clues, depending on the answers.

At this point I am very suspicious of the chamber dimensions.

Please let us know what you find out from a gunsmith.

Regards,

Tom