Definitely not a squib. Had it been a squib, the barrel would have been destroyed at the point of obstruction.
As to how this can happen with a locking, rotating bolt, it's easy: Remember the system of operation of the AR/M16 family. When the cartridge is fired, at a certain point in the bore (in this case, near the muzzle) the bullet passes a gas port. The expanding gases head into the port, down the gas tube and into the bolt carrier key.
(Bullet is still in the bore.)
The gases, still coming back with full force and pressure, head down the carrier key and into the back of the bolt carrier--into the small space between the back of the bolt and the bottom of the bolt carrier. This gas is still under pressure, so it gets packed into that small space. Now, something has to give--in this case the carrier starts sliding backward.
(Bullet is now exiting muzzle. Gas pressure is now dropping at a REALLY fast rate.)
The carrier's rearward travel causes the cam pin--which is anchored solidly through the bolt--to rotate, taking the lugs out of engagement. Now, the bolt carrier is still traveling rearward under a bit of pressure--which is vented through the two holes in the right side of the carrier.
(Rifle is now completely unlocked. Pressure has dropped to zero. Something is in the process of getting acquainted with a 5.56mm bullet moving REALLY fast.)
The potential energy of the expanding gas has now been converted completely into kinetic energy. The bolt carrier group moves backward under that energy extracting and ejecting the brass. The kenetic energy of the fired round has now been overcome by the resistance of the recoil spring in the buttstock; the BCG now moves forward; feeding, chambering and locking again in preparation for the next shot.
So, what happens when all of the above takes place with a long bullet that has been jammed back into the case upon feeding?
Remember that the case is a pressure vessel--when you decrease the amount of space inside then the same amount of gas will produce a much higher pressure. In this example, all of the above occurs--but the gas hitting that BCG has TRIPLED in pressure. Oh yeah, it's going to unlock--HARD. That carrier will come roaring back. Fortunately, there's a vent there--the mag well. All of the excess gas will head straight down that mag well. Yes, you'll get an ouch--but with a properly manufactured rifle you may well be OK. If you are stuck with an inferior or flawed bolt or bolt carrier, the metal might shatter like glass. NOW you have serious problems.
Hope that all helped, folks! :hello: