This is a question regarding barrel safety issues on delayed blowback and gas operation systems.

In a gas operated system,the barrel as a gas port drilled so gas can vent to operate the piston to unlock the bolt,before the gas can vent to it the gas is trapped between the locked bolt and bullet,if something stops the bullet as the gas expands this causes gas psi to raise and it blows up the barrel as this pressure excess is more that to what the barrel was designed to hold at that point.



In a delayed blowback,there is no conventional gas operation,it used Newton's third law of motion so as the gas expands it moves the bullet out one way and uses the case as a piston to move  the bolt in the opposite direction,a STG-45/CETME/G-3 bolt uses mass and a roller mechenism to delay the bolt rear motion so it can open at safe pressures.
As the bolt does not lock and as the nature of the system is to have the bolt moving back (as it fires),if something stops the bullet and gas pressure keeps raising,can the barrel blow up or does the pressure excess just slam the bolt back to open?







In Semi-Automatic the bolt strips and chambers in to a unserviceable barrel but in Automatic it keeps striping and chambering and firing in to a unserviceable barrel?

What system is safer?

Does the excess pressure causes the bolt to open faster or does the barrel explode?

Was the ArmaLite AR-10 n#1002 barrel failure only due to the composite material lack of resistance?

In answer to your question, if the bullet can't exit the bore before the delayed blowback action has moved back far enough, the case head of the cartridge ruptures as extraction is attempted and looses that high pressure gas inside the receiver inches from your face, usually destroying the receiver and mangling the bolt and blowing the rifle apart in the middle instead of out at the barrel end.

Safe operation is always dependent on the bullet having exited the bore and pressures inside having dropped to safe levels before any unlock and extraction is attempted.

Something like this?

Exactly like that.  Can happen on blowback guns, too.

This one was possibly  a result from a Cook-Off?

I don't think so.  A cook off is not inherently dangerous to the firearm, it's just the ammo heating up enough to ignite the powder and fire the weapon without the user operating the trigger.

But a Cook-Off can cause a case to fire as the bolt is locking,as the resulting gas expansion in the receiver damages it.

"View of various damaged components resulting from cook-off out of battery"

The two images show two rifles with diferent levels of damage,the first image the bolt exploded and the upper receiver is in pices,the second image shows a crack in the receiver,as it used the Bolt Closing Device and the top of the receiver was machined so it could use part of the mechenism the receiver was less strong and cracked,the two may show the result from diferent incidents but the two show the problem of having gas expanding in the receiver.

I won't dispute the conclusion of those who took the "cook-off" photo, but will say it's a far, far extreme case - as in you were hosing out hundreds of rounds in a row until the barrel glowed to get it hot enough to cook off before the bolt could close.

A typical cookoff scenario is that you fired a couple hundred rounds in combined auto/rapid semi, within a minute or two (literarily) in a very intense firefight.  The barrel is now extremely hot up front, and the heat is working its way back towards the thick breech/chamber area by conduction, and maybe 15 seconds to a minute after you stopped firing, with a round fully chambered, the chambered round goes off from the heat leaching into the case.  That round will fire, and another will load if there's one left in the mag, and that one might cook off in another 15 seconds to a minute.

The first, catastrophic KB pic you showed is clearly from gross overpressure, just as the second (labelled "cook-off") was clearly an OOB/head separation… which can result from bad ammo or a high primer, or debris on the bolt face, or, apparently, a "cook-off".

ETA to fix "cook off"

The reason for the question was to know if a delayed blowback can act as a "gas valve" so the trapped gas can vent out the barrel,so the barrel does not blow up,but the rapid and abnormal raise of gas pressure causes the bolt to open prematurely,so the gas pressure that does not blow up the barrel blows up the receiver?

This started after watching the Fairchild Armalite AR-10 presentation film,when E.Stoner places the second model prototype AR-10B inside a box filled with mud (for a mud test) then takes it out and fires it,and it fires with mud in the barrel.

Doesnt the mud in the barrel slow down the bullet as gas pressure raises up?

Yes - but in a gas system, there's no pressure in the gas tube/bolt carrier until the bullet makes it past the gas port, which in an AR-10 or AR-15 is way down at the end of the barrel.  With bore obstructions, it becomes a wonderful hydrodynamic stew of the still-burning expanding powder gases behind the bullet, and the relative drag of the bore obstruction, and the dynamic strength of the barrel steel.

A bore obstruction can either:
1) do nothing harmful - the obstruction is pushed out by the bullet, nothing rises to supercritical levels, the firearm functions as normal
2) do no immediate harm, but contribute to ongoing stress - the obstruction causes abnormal wear on components, without outright breaking anything - it will now wear out sooner or more easily than otherwise, but will continue to function normally
3) do immediate, non-catastrophic harm - the obstruction exceeds elastic limits locally and produces something like a "ringed barrel" from a squib bullet, but otherwise remains safe to fire, and operates normally, albeit with perhaps a reduction in accuracy or efficiency.
4) do immediate, catastrophic damage - the obstruction causes a component to go supercritical and fail destructively, after which the firearm will not function again without repairs.