Could this be the cause of blown extractor?

[c_jit]

After I have reduced the trigger pull to about 5 lbs. using surplus USGI M16 hammer and trigger springs as described >>> Here <<<

I have collected some 50 spent cases to check firing pin strike depth for consistentcy.

I line them the cases so that the firing pin marks are in the 12 o'clock position.

ความคิดเห็นที่ 96

I have noticed that all the cases are very dirty and all in a particaulars area.

So, I fired a few cartriages from the same box but from a different gun.

I then compared them side by side.

Three spent cases from GSG5 are on the left and Cases from MP15-22 are on the right.

They are from CCI mini-mag HP 36 grains.
Top side image

I randomly pick some more spent cases fired from my MP15-22.

I checked the bolt face and found what I think an excess blow back gas trace toward 10-12 O'clock direction.

I know it is a blow back design and supposedly dirty from the gas.
However, I think some batch of barrel has very sloppy chamber cut and thus causing gas leaks.

Some symptoms of a very loose chamber is a blown extractor or in worst case, a ruptured case which were reported as OOB by some members.

This is why S&W asked you to send the whole gun back instead of just the bolt.

May be it is the new barrel that they are putting on, not the extractor as were previously suspected.

For your info. my 15-22 is DTM0730

 

[chuck s]

I'll try to recover a handful of cases today or tomorrow. I did the "15 minute trigger job" as well but have only fired a couple of rounds.

A lighter hammer spring will allow the bolt to be blown back faster than a heaver one and cases will be "extracted" (they're really blown out) sooner in the firing cycle with more pressure remaining in the barrel. Not sure this would blow a case sidewall though as the high speed photography I've seen has the bullet exiting the barrel before the bolt moves and the path of least resistance for the gas is out the muzzle. Detonation with the case only partially chambered, though, is trying to push the bullet down the bore and the pressure could rupture the case.

Years ago when I was much younger and a little more foolish we tested .22LR cartridges for out of firearm detonation. Cleaning patch soaked in rubbing alcohol, cartridge on top, match. All in a heavy cardboard box. Light. Close the lid. Bang! Bullet comes out. Case had no major deformation.

-- Chuck

 

[maxicooper]

Very interesting indeed !!!

Thank you for sharing.

 

[UtahHunting]

I couldn't tell you if that is the cause or not, but this is certainly an interesting observation! I will keep an eye on this thread to see what others have to say.

 

[Connor9000]

I would be inclined to believe this theory. I just sent my 15-22 back to S&W for repair. I marked the parts and noticed they changed my entire upper out and swapped out the trigger parts on my lower. Mine was double shooting. But i noticed the powder burnt cases and my bolt face wasn't square to the barrel. There was a small gap at the bottom of the bolt. It isn't there anymore. So your theory has merit.

 

[rip3000gt]

When Mine had an OOB I looked at the shell and there was no firing pin mark on the back of the case, so my OOB some how the rim got crushed by the bolt when it reloaded

 

[ChattanoogaPhil]

I know it is a blow back design and supposedly dirty from the gas. However, I think some batch of barrel has very sloppy chamber cut and thus causing gas leaks.

11k rounds with Fed Val Pack and my 15-22 hasn't had any cycling problems (other than mag-snags), and no FTF either. However, your observation regarding bolt grime is identical to mine.

I usually shoot 500rds+ with the 15-22 during a range visit. The bolt face has significant caked build up. I shoot my Ruger 22/45 every Friday and put about 200-300rds thru it. I clean it on the first Friday of every month so it's got well over 500rds through it now. The bolt face is pretty clean.

The amount of 15-22 crud build-up reminds me of my .45ACP when I reload with very light loads. The brass doesn't expand properly to seal the chamber so things get dirty quick.

There are a lot of folks here who have sent their rifles back to S&W. Maybe they can say whether or not they've noticed any difference? Just as dirty or running cleaner?

On a rambling side note..... the 15-22 bolt is HUGE. Could it be that the 15-22 bolt has greater inertia than most other .22s and that when a round doesn't want to fully chamber the rim gets crushed more resulting in OOBs?

 

[27 Beck]

Ok, I have lost two extractors to OOBs. Prior to the OOBs I would have failure of the spent cartridge to clear the gun, causing jams. Seems that it would be possible for a spent cartridge that has not cleared the gun to be hit by the bolt coming forward and causing the chambered round to be ignited. So instead of the firing pin hitting the chambered round the spent cartridge is driven into the chambered round by the bolt coming forward.

 

[c_jit]

Ok, I have lost two extractors to OOBs. Prior to the OOBs I would have failure of the spent cartridge to clear the gun, causing jams.....

This is another sign of severe gas leak that a fired case did not seal properly and retain enough blow back pressure to propel the bolt completely.

 

[chuck s]

See this thread. New this morning:

http://smith-wessonforum.com/smith-wesson-long-guns/119681-m-p-15-22-4-3-4-mags-then-boom.html

-- Chuck