There isn't enough gun powder in that pipe bomb to "rip off" the upper. You have to have some serious shear forces to overcome the metal in the barrel extension and also the receiver. That much powder won't make it happen. The occasional blown out .308s that have made their way to the Interweb have suffered only minor damage similar to this. The M14 that had a receiver failure years ago was attributed to poor metal for the receiver that caused defects. That is the exception.
Unless yo'ure talking catastrophic failure of a .50. That may do it. Catastrophic failures that rip the metal receivers to shreds would require dynamite-level charges... I read about some booby-trapped ammo we placed in Vietnam used in enemy AKs and those caused legendary failures.