Starting to see a theme here with the Hornday ammo.



Ammo is showing signs of over pressure, and the what you seeing is the primer blow out of the primer pocket when the bolt cycled back, and the marks on the rims of the spent case are also screaming that the round when over pressure as well.  Hence rim flowed into the ejector channel area, and the other side of rim bent to hell from the extractor having a hard time pulling the spent case out of the chamber when it went over pressure as well.

Get Hornady on the phone with a lot number to go over the problem of the ammo going over pressure, and stop shooting the ammo through the rig.  At this point, really don't know if the ammo is just nato working pressure and some are trying to shoot it through a none 5.56 nato chamber to cause the over pressure problems, or if the problem is the ammo is loaded to even hotter than max nato at 65K for even in a nato chamber.
Note, LC M193 is only loaded to 62K, while some other import nato is loaded to 65K instead.  
The hotter ammo works fine in a true mil spec Nato chamber since they tend to be looser side walls to promote reliability that allow more blow-by at fire forming, while some chambers that are reamed nato in the leade, have tighter cross wall dimensions to promote accuracy instead.  So what may run fine in a say a Colt or FM Nato chamber, may end up going over pressure in a tighter reamed sidewall chamber nato labeled chamber instead.  Hence in both 223 and 5.56 nato, they have the same head space distance from the bolt to shoulder of chamber, but The leade is tighter on the 223 chamber isntead.

So we have this for last of chamber leade cuts,



And the last part of the match is the side wall dimensions, with USGI chamber a touch more looser in the side wall dimensions to promote reliability during higher fouling levels, while civilian chambers tend to be tighter in the cross wall dimensions to promote accuracy instead.

Look at the C base line numbers, and keep in mind that the Colt and FN chambers are a touch looser than the .3816" number.
Note, the reason behind this, is the FM and Colt USGI barrels are chrome lined, and cross wall over sized so they don't end up too tight after chrome plating since there is a slight variance on plating build up that you can't control.