Pressure Maybe?

[tp555]

Customer had a problem.Total lock up.Worst I have ever seen.Bolt,carrier and upper were not coming apart.Load was 12g of 296 w/ 240 gn bullet.He chrono'd
it at 1200.The only shot.Barrel was 10 1/2" SS one of mine.Extractor was bent.I did get it apart by cutting the barrel extension.

[Hoser]

With the blown case, extractor dimple and blanked primer I would also guess excessive pressure.

How does the inside of the barrel look? Maybe ringed from a stuck bullet in the previous shot, or when you say only shot was that the first round he fired?

[Colohunter]

Looks like pressure to me as well, looks like Sierra's manual shows a maximum charge of 296 as 11.3 grains for 1200 fps, Sierra shows that data was from a 10" T/C barrel.

[LouBoyd]

What was the cartridge OAL? Seating depth makes a huge difference to pressure in a tiny cartridge like the 300 Whisper. The bullet can be seated much further out in a T/C. The default Quickload seating depth for the 300-221 of 2.3" is too long to fit in an AR-15 magazine.

I just ran a Quickload simulation on a 300-221 with a 10.5" barrel with 12 grains of Winchester 296 with a Sierra 240 grain Matchking seated to a cartridge OAL of 2.28" using defaults for other information not given in this thread. It predicts a pressure 59140 psi and muzzle velocity of 1373 fps. That's just a bit warm for an AR-15.

With that seating depth, bullet and powder I wouldn't have exceeded 9.0 grains, which Quickload says would give 1016 fps and under 28000 psi.

[tp555]

Don't know the load details.The brass flow into the ejector is high pressure.I got the bolt and other parts separated today.The chunk is at the extractor section which was bent on the bolt.I'll try to save the barrel.See what happens.

[BWE Firearms]

That is defiantly excessive pressure. Check the barrel out very carefully before trying to save it. I would also suggest Magnufluxing it. Make sure the receiver and bolt carrier didn't sustain any damage as well.