Since you don't know what happened, odds are pretty high that it was reloader error.
Maybe you forgot to empty the hopper completely and had a full case of pistol or fast rifle power? Maybe you didn't size a case properly, maybe you used too slow a powder and too light a bullet and had a SEE?
For your little primer test, how much bolt lug contact do you think is still safe? If there is 3/4 contact and the primer fires is that safe or catastrophic?
Out of battery means no locking at all, and I will reiterate it's pretty impossible to fire an M1 or M14 completely out of battery due to the design. Maybe you should try your own test and tell us how much lug engagement you have when the rifle is still able to fire a primer.