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Old March 15, 2011, 08:48 PM   #20
Webleymkv
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Join Date: July 20, 2005
Location: Indiana
Posts: 9,831
Originally posted by Jim Keenan
Quote:
I think that at one point, the bolt or the carrier stopped its forward motion, short of battery, for some reason. The shooter, not realizing the bolt was not fully closed, pulled the trigger. (That gun does not have a disconnector.) Some defect of the gun allowed the hammer to reach the firing pin; the extractor held the case head firmly enough to ensure good support for the case and permit a full firing pin blow. And ka-boom!
I'm not sure if that's possible as, if the gun weren't fully into battery, the hammer should strike the back of the bolt carrier rather than the firing pin. The only way that I can think of that the above scenario might have happened with my rifle is if the hammer striking the bolt carrier went ahead and shoved it the rest of the way forward and struck the firing pin on the way. In that case, however, I'd think that it would also go ahead and push the bolt into battery thus preventing the situation that occurred.

Quote:
The case seems to confirm the above theorizing. The case blew out, but enough was in the chamber that high pressure gas came back around the case, crushing it. The internal pressure meanwhile had blown the head off the case in the classic "excess headspace" condition. There was probably enough pressure to push the bullet out the barrel; the OP doesn't say, but I think he would have mentioned a stuck bullet.
Nope, the bore was clear.

Originally posted by wogpotter
Quote:
I agree about the firing pin impression, what bugs me is the line from the impression over to the edge of the primer!
That looks like the pin was pressed into the primer when the bolt unlocked (or locked) I can't tell which, but I'm favoring locked because of the center hit on the primer cup.
If that's the case we're looking at a gas system malfunction not having enough dwell time before unlocking, or a shot locking shoulder or bolt locking face.
A gas system issue is possible as last time I adjusted the gas system I was using a different brand of ammo (Federal 150grn Power-Shok). I notice from looking at the manufacturers' websites that the Winchester ammo I was using (150grn Power Point JSP) is listed as having a muzzle velocity 10fps higher than the Federal. I suppose it is possible that the Winchester sent more gas through the port than usual and caused the bolt to unlock prematurely. However, in that case I'd think it would be more likely to simply fail to extract and/or tear the rim off the case.

Originally posted by Jim Keenan
Quote:
I see what you mean about the line, though. It looks like what you might see in a dropping barrel pistol, but the FN-49 bolt doesn't drop at the front. The only way I can see that line happening at the time of firing is if the bolt was actually blown upward or to the side, leaving that mark as the firing pin scraped the cartridge case. What happened to the bolt and bolt carrier? Did they blow out of the gun?
No, the bolt and carrier were in their normal channels immediately after the incident.

Quote:
Also can you tell where the extractor was? Those look like extractor scrape marks on the (viewer's) right side of the picture. The extractor position would let us orient the case the way it was in the gun.

A thought, though. One thing that could stop the bolt carrier and the bolt would be the gas piston if it stuck out part way. Can you check to see if it might be bent or have a burr or something that could do that?
I can't really tell where the extractor was on the case. I can tell you however, that if you are looking at the face of the bolt from the front, the extractor is on the upper left-hand side (the rifle ejects up and to the shooter's right). Also the gas pistol was not stuck out immediately after the incident and moves freely as far as I can pull it against its return spring.

Originally posted by wogpotter
Quote:
I replaced the 1-piece firing pin in my FAL as they have similar pin breakage issues. After swapping them I had too much FP protrusion with "identically interchangeable" parts! I ended up using a FP protrusion gauge & re-grinding the FP front half's tip, but only after a lot of checking to find where EXACTLY the discrepancy was. Luckily I discovered the problem before there was a problem.
I have not made any modification to the firing pin since it was installed. However, the pin does not protrude from the front of the bolt unless the rear portion is depressed. Also, I replaced the firing pin return spring with an extra-heavy one after having issues with slamfires before (it would fire 2-3 shots with 1 trigger pull).

Also, I considered maybe a problem with the sear or auxillary sear engagements allowing the hammer to follow. However, none of those engagements appear worn nor was I able to make the hammer follow in messing with it unloaded at home. In order for the bolt to travel rearward enough to feed a new cartridge, it always seems to ride back far enough to re-cock the hammer no matter how I try to manipulate the rifle.
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