It also said the cracks were old. 3500 rounds is nothing, for a semi auto rilfe.
It was also clear that they do not know if the bad ammo cuased the failure or if it would have failed other wise.
Also look at the pics of the primer, when guns blow up there is less persure in the chamber, not more( obstucted barrel is a diffrent story) ITs clear that there was a over persure in the chamber.