The barrel definitely wasn't rotated 45 degrees, more like 20. The gas block was still located properly over the gas port, and the gas tube was still in the chamber. Unfortunately I don't know the sequence of events as I wasn't there.
The pinned gas tube would have applied spring tension to prevent it from rotating TOO far out of spec.
Not sure the total round count, but this was NOT the first round through the gun. Headspace was checked before, and then after the failure and is good.
Makes sense, the barrel extension is still in the same place it was before.
The round before this round was a light primer strike, didn't go off. Rifle may or may not have been in battery, nobody noticed, they just ejected the round and then fired the round that led to the KB.
Which really make me think it went down like I said in the last post I made. ApacheTodd's post got me thinking more about what would happen if the barrel were allowed to rotate freely. Due to the torque pressure of the nut, and the later spring "tension" provided by the gas tube.
Imagine that on the LAST round that was fired, PRIOR to the light primer strike, the rearward motion of the bolt starts. The lug is already sheared (has been weakening over time due to the void.)
The gas tube is still lined up, gas impulse arrives at the key. The bolt carrier begins moving back, rotating the bolt; but there's pressure against the lugs, and the barrel STARTS to rotate - just a little - with it.
The round ejects.
Bolt carrier starts moving forward.
Picks up a round.
But this time the lugs are out of position. When the front of the bolt face reaches the lugs, they don't line up; so the bolt face starts to CLOSE on the bolt; rotating due to the slant cut in the carrier.
Suddenly SNAP they line up and the momentum carries the bolt forward a bit.
But it lost enough momentum and it's badly out of battery.
Shooter fires - no boom! It's not far enough forward at the time of the hammer strike to transfer energy through the firing pin. However the bolt carrier budges forward a touch more.
Shooter yanks on the charging handle. Bolt carrier moves back - but the bolt lugs are in a freely rotating barrel. Barrel shifts over MORE.
Next round is chambered. Bolt carrier and bolt goes forward, bolt face impacts lugs (again!). Bolt begins twisting - doing it's job when the bolt gets rearward pressure. Lines up with the lugs, slips in to the chamber.
At this point the barrel is rotated nearly one full lug from it's original position (say, 20-30 degrees).
The bolt carrier and bolt are far enough forward this time to allow the hammer to transfer energy properly to the firing pin.
BOOM.
Cartridge goes off. IMMEDIATELY starts to get thrust OUT of the chamber, NO RESISTANCE!
Case ruptures after 1/2 inch of rearward travel. Crap starts blowing apart.
The bolt itself receives such a nasty hit that things inside start bending and twisting. The retaining pin is gouging in to the receiver and ultimately stops the rearward travel of the bolt.
(Good thing too, otherwise it might have been a nastier failure).
It's possible that the round was overpressure, or that the index pin had already snapped, or both, and upon firing the hot round the barrel rotated just after ignition allowing the carrier to move to the rear to early while pressure in the chamber was still high, which would then rupture the case and lead to what happened.
I seriously doubt it was caused by ammo. You have a failed part. And a solid theory now of how the failure occurred.
In this case torque on the barrel would be opposite to bullet rotation as that's the only source of torque just after ignition an initial bullet travel. This at least agrees with what I saw... right hand threads, barrel rotate approximately 20 or so degrees left. Equal opposite reaction and all.
It wasn't the bullet's twist that did it, it was the way the bolt twists within the bolt carrier. When the bolt starts moving backwards, there's still significant backpressure between the lugs on the bolt and the lugs on the receiver extension. The pressure drops quickly, as the bolt carrier diagonal cutout causes the bolt retaining pin to rotate the bolt and clear the lugs.
The bolt doesn't look any worse for wear but the carrier is definitely done.
That bolt took one HELL of a hard hit. I'd be wary about using it again. The LUGS would be fine. But man, the shearing force it was under around the bolt retaining pin would have been frigging incredible. I've had bolts snap in half there before, it's a weak spot. (But not a bad failure, generally doesn't hurt anything else when it happens).
I'm not sure that the barrel could have been rotated before the KB, seems less likely, but I'm sure it's possible. I'm most perplexed by the snapping of the index pin and the rotation of the barrel. Any thoughts? Overpressure round? Bad index pin? At 55 ft-lb of torque would the barrel rotate without the index pin under otherwise normal circumstances?
The barrel was almost certainly rotating for at least one, and perhaps many more shots prior to that kaboom.
Upon closer inspection of the broken index pin in the barrel extension, its face appears to have an internal void, such that it would have been more prone to snapping.
Then, it was just a matter of time before the part failed. This stuff happens. We don't x-ray our gun parts, generally, to find hidden flaws. (I've seen old black powder cannoneers x-ray their cannons before, but those are odd folk.)
In any case, where did the torque come from that lead to the snapping of the index pin and the subsequent barrel rotation? The muzzle device was installed with shims, not a crush washer, and install torque was mild, maybe 20 ft-lb. The pin should have been fine after install and the barrel was straight.
The bolt itself.
