Lets start from the top.

Bolt locked up, the primer is ignited, and as the ammo is firing forming to the chamber walls, the case will expand to seal off the chamber via the case.  During the firing forming, some of the pressure is lost out of the back of the chamber.

So if the ammo is over sized, then as the round is first being loaded into the chamber, often the shoulder of the case will seal off to the shoulder of the chamber (wedged in as the bolt is locking up), and when the round is ignited, less pressure loss at fire forming to the chamber, more pressure down the barrel, and this cause too high of working pressure of the round itself (both over all, and gas pressure at the gas port).  With ammo that is loaded to normal working pressure (just the case over sized, the pressure is not so high that you have to worry (less then a proof load), but still enough to cause short stroking instead.


Now since your bolt did not unlock at all, then you have other problems going on instead.

The first may be that  the chamber was not scrubbed clean to begin with (chamber brush and CLP by hand), the head spacing it too short between the bolt and chamber, or the chamber has nasty reamer chatter instead.  These will cause problems with the bolt/extractor being able to pull the spend case when the resdual pressure in the bore is around 20K PSI at unlock.

Or, could be that you have a leakage/blockage in the gas system, and no allowing enough gas pressure to the B/C gas chamber to unlock the bolt in the first place.


So step one, clean the chamber with a chamber brush and CLP by hand, and once clean, take a good look at the chamber to check the chamber wall conditions (should be smooth as a mirror).  

Next is to check the head spacing with a head space gauge.  

From here, start checking the gas system for leaks.  On this, easy enough to put CLP between the gas block front and back seam to the barrel,  CLP between the gas tube to block, then using a piece of tubing on the end of the gas block in the receiver, 100psi of compressed air down the gas tube to see if the block to barrel/gas tube to block has a leak.   On the B/C, CLP around the base of the key to top of carrier, then hold the bolt inward as you push 100psi of compressed air through the front of the key.  On all three seams, you should not have gas leaks, and even through the air pressure will flow through the barrel gas port and into the barrel bore, the restriction of the barrel gas port is enough to check the gas block for it leaks.

Also, with the bolt off the carrier, use the carrier with key dry fitted in the upper receiver.  This will tell you if the gas tube is correctly aligned with the key, and if the key base is too wide for the upper receiver slot (carrier should not bind in the upper receiver).