I have investigated at least five incidents of what appeared to be OOB firings involving both M16A2s and M4s using GI ammo and was never able to pin down an exact cause. In one case I was on the scene just a few minutes after the incident (at Robins AFB GA in 2007) and was able to capture all the evidence before someone else screwed it up. It involved a female airman and she was really shook up. Unfortunately, the shooter's memories of exactly what happened were not good due to emotional trauma. At least there were no shooter physical injuries associated with any of those incidents. I remember that in two situations, the cam pin was fractured at the firing pin hole (presumably as a result of the blowup). None resulted in what I'd consider catastrophic damage to the upper receiver or barrel. It's been too long ago for me to remember much specific detail. I used to have pictures of the damaged guns and components, and may still have those stored on a hard drive somewhere. I'm not going to look for them.

For sure, an AR action will NOT fail under excessive pressure if it has complete lockup of the bolt head into the barrel extension. Colt has demonstrated that using cartridges having extreme overloads of fast powder, producing peak chamber pressures of well over 100,000 psi. And to a lesser extent, so have I by once performing a series of experiments involving in-bore collisions using bullets stuck in the barrel at different distances from the chamber. All that ever happened was barrel bulging, never action failure or barrel splitting. Therefore, an AR blowup must be the result of an OOB firing condition. The unanswered question is how did that happen? And there may be multiple reasons.