We still need pictures!

Well, I finallly called a friend with several decades of experience with these things going back to 'Nam.  He told me that the KB almost certainly occurred due to high pressure (whatever the reason), but that the case head ruptured while action was locked, as many folks (brighter than me) suggested.  He also agreed that it was not OOB firing.  

He felt that timing in gas guns was more of an extraction reliability and parts-wear issue.  

I had not realized that the AR's extractor cutout was relatively unsupported, and couldn't otherwise make sense of PJ's findings.

Mea Culpa.  Sorry to have contributed to the confusion.

Sam

Quoted: ...He told me that the KB almost certainly occurred due to high pressure (whatever the reason), but that the case head ruptured while action was locked....

From the Kbs that I have read about, that is pretty typical.  The case virtually always blows out under the extractor, and the extractor is usually bent outwards.  And even though the bolt is locked, the case still expands out radially at the (very small) joint between the bolt and chamber.  That is what causes the 'belt", and often, complete case head separation.  The bottom of the bolt carrier  typically blows out  too.  Once that happens, the receiver bulges outward.  The pressure then blows down through the magazine well.

Here is another recent Kb that has almost identical damage.  

See my newest thread...........I think I know what and why.

PJ

Quoted: Argggghhhhh.......... We have incontrovertible evidence that PJ's gun KBd after unlocking.  The evidence is a ruptured case head - and intact lugs and chamber.  How in heck could the case rupture if it wasn't under pressure?  How in the heck could the chamber be intact if the case ruptured while the action was fully locked?  If the bullet - or something - weren't still in the barrel how could the case head have been made to rupture?   Think about it.   Now, if you're still with me, let's go further:  what do you think would have happened if this round had been fired in a gun that had it's gas tube disconnected?  Answer:  either the lugs/chamber/barrel would have held (= no KB, no unlocking), or parts would not have held (= KB).  In this case, though the KB would have involved sheared lugs, ruptured barrel, ruptured chamber..........one, some, or all of the three. The reason some of you have trouble seeing what happens with a high-pressure KB in an AR is b/c you're viewing a dynamic process with static vision.  Under normal circumstances the gas operated AR does not unlock until the bullet has left and pressure has dropped.   Yes, we all know this.  Please listen to what I'm going to say:  this was not a normal circumstance.  Whether due to excessive case length........or whatever...........this was not a normal circumstance. If you have an AR close by this is a good time to take out bolt assembly.  Now, put the lugs in the locked position (carrier all the way forward).  Next, start to move the carrier backwards, as occurs in normal operation.  You just noted that the carrier moves a bit before the lugs start to rotate.  Ahhhh..........the word we're looking for is "timing". All automatic cartridge weapons must be timed.  All of them.  Period.  Doesn't matter whether they're recoil, delayed roller, direct gas, op-rodded, eieio........whatever.  All systems use cartridge energy - whether as recoil, or as diverted gas.  This energy - in whatever form - must be used to move parts of defined mass.  It is the job of the designer to make sure the gun stays locked until the bullet leaves and pressure drops.  But the designer also has to be sure the gun will actually work.  It must cycle.  What to do?  Answer:  design the gun to function properly within certain a pressure/energy/recoil range.   Now you all understand that a pressure overload of your Ruger 10/22 can result in a case failure.  We see the same thing, from time to time, with MACs and Uzis.  The case would fail during opening.  If pressure were somehow much, much more excessive the chamber could KB along with the case before much movement had occurred. Now let's look at a gas gun.  What do you think would happen if you made a bolt/carrier assembly entirely out of titanium.  Let's throw in a very soft recoil spring.  And just to show you what a nice guy I am let's install this in your AR (not mine) and use plain Jane factory ammo.  Right, we won't specify a pressure overload.  What do you think would happen?  Right.  Early-opening  and possibly a case head rupture.  Why?  B/C the system was improperly timed resulting in premature opening.   Back to PJ - no he didn't have a titanium B/BC, but he most certainly had an over-pressure situation.  Result was too rapid movement of bolt carrier with consequent unlocking before pressure had fallen. And now you know why guys like Stoner aren't dummies.   Sam

WOW!  Thanks Mr. Hoggson.  Extremely educational.

Ok now.....Lets see if this will fly.
Support of the bolt opening under pressure is one that I was reluctant to consider. Then I read other kbooms and the results are similar.

There are now two ways the bolt can be opened in my opinion. One is for normal operation where the gas opens the bolt. We all understand this.
Now, it is clear the case blew first due to an obstruction of some type, now the bolt unlocks and blows the mag etc.

If the pressure is great enough to blow the mag and bulge the lower, then that gas venting directly inline with the face of the carrier was sufficient to move and unlock the bolt. The extractor couldn't move the case under 50,000 psi.  

Do any of the members have contact with any of the ar 15 Manf. that can evaluate this possibility.

If this is what happened then it could happen to any of us. Jim

Were you bump firing underwater AGAIN???  Sorry but I haven't seen that reference in months and I couldn't resist.

Quoted: Were you bump firing underwater AGAIN???  Sorry but I haven't seen that reference in months and I couldn't resist.

Don't see the humor in papajohn having $500.00 damage done. Maybe you do.