The velocity of the bolt when it taps the back of the receiver with normal unlocking is... "very low" in comparison to the velocity it hit in this case.
But your point is well taken. "Low" is relative in comparison to the loads at the locking area up front, and is also "low" in the normal case as compared to this particular case in the rear of the receiver.
I will edit my previous post to more accurately reflect this, and continue the explaination below to hopefully avoid any errors. Thanks for the excellent point.
My bet is that the op rod was not connected to the bolt when this occured.. depending on how the rod bent it likely disengaged the cam. The back of the receiver then took all of the load... no op rig spring or the mass of the op rod to slow things down. The rear of the receiver is what prevented the bolt from entering the face of the shooter and exiting the back of his head. No kidding. Even if the op rod stayed attached, this was a VERY bad thing to have occured, see following:
Think about shooting an "unlocked BLOWBACK .30-06" rifle" v/s a "Locked until bullet passes gas port at end of BBL and then a delay while to op rod cams rotate the bolt, and then retard that with op rod spring and the mass of the op rod/bolt combination" rifle and my guess is that the forces at the rear of the receiver were likely an order of magnitude (10X) those normally seen there, or perhaps even more. Maybe 2 orders of magnitude (100X). It's a REALLY large difference.
Evidence: What we can surmise by the description of the recovered case is that the round was NEARLY chambered when this occured, otherwise the case would be ruptured and likely fully separated at the head. A fully unchambered premature would have developed low pressures. THIS thing developed what is likely nearly FULL pressure in the case. Blowback .30-06 with a LIGHT recoil spring being the only thing resisting the load? Yowie!! The MOST VULNERABLE point for a premature is JUST before the bolt begins to rotate to lock, IE: the round is "nearly" full chambered so it will develop full pressure, but nothing is holding the bolt locked. I would be interested to look at the bolt to see if there is any evidence of any locking... even a few degrees. None PLUS the evidence provided by the case shape = worst case from a stresses standpoint.
I'd still be inclined to play with it... but with the tools and techniques here to do this correctly at no cost other than labor. The OP probably does not have that facility. For HIM the best bet is likely to buy another service grade rifle. HERE I'd look it over more carefully and likely salvage it.
Last edited by Willie Sutton; June 2, 2012 at 09:38 AM.