Author Topic: Help me understand what happened with my 9mm  (Read 8416 times)

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Offline DOC 1500

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Re: Help me understand what happened with my 9mm
« Reply #15 on: June 07, 2020, 10:15:40 AM »
What is the marking at the very base of the Bulge
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Offline Texas377

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Re: Help me understand what happened with my 9mm
« Reply #16 on: June 07, 2020, 01:09:34 PM »
Definitely out of battery.  No Detective Columbo needed here.  That is a scary looking casing.   :o
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Offline Tok36

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Re: Help me understand what happened with my 9mm
« Reply #17 on: June 07, 2020, 01:13:33 PM »
What is the marking at the very base of the Bulge

   I was looking at that mark as well Doc, i can not figure out what it is. First i was thinking maybe the extractor, but it is too wide. Then i was thinking maybe the bolts breech face, but that dose not seam right ether due to the marks shape. A mystery indeed.
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Offline SI VIS PACEM PARRABELLUM

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Re: Help me understand what happened with my 9mm
« Reply #18 on: June 07, 2020, 04:12:56 PM »
What is the marking at the very base of the Bulge

   I was looking at that mark as well Doc, i can not figure out what it is. First i was thinking maybe the extractor, but it is too wide. Then i was thinking maybe the bolts breech face, but that dose not seam right ether due to the marks shape. A mystery indeed.

I bet if we saw a good enhanced pic of the breechface there would be some machining in there that matches up to that pretty well.

Offline George16

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Re: Help me understand what happened with my 9mm
« Reply #19 on: June 07, 2020, 06:44:47 PM »
It seems that the there are two marks opposite each. I agree that it might be the breechface clamping on it when it detonated out of battery.

Offline Tok36

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Re: Help me understand what happened with my 9mm
« Reply #20 on: June 07, 2020, 08:27:23 PM »
   The rifle used was a CMMG 9mm radial delayed blowback. The rear of the locking lugs have tapered cuts to facilitate unlocking but the bolt face looks just like an AR15 bolt. In the brass pictured the depression has defined ends. I would figure if the bolt face left the depression it would have tapered ends because the case was at an angle or go all the way round the case if it was inline with the bolt.


« Last Edit: June 07, 2020, 08:31:41 PM by Tok36 »
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Offline Crawl

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Re: Help me understand what happened with my 9mm
« Reply #21 on: June 07, 2020, 08:50:48 PM »
It seems that the there are two marks opposite each. I agree that it might be the breechface clamping on it when it detonated out of battery.
Not sure what you mean by clamping. Here's a photo of the bolt.

By the way, I was able to repeat the issue this weekend during extremely slow fire.

All the various engravings at the base of the neck, just blow the donut are the inside edge of the bolt. The gaps are from where the ejector and extractor are. Also there one section on the "bottom" of the bolt face that is cut out.

Offline Tok36

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Re: Help me understand what happened with my 9mm
« Reply #22 on: June 07, 2020, 09:16:35 PM »
   I see, i did not know about the extra cutout at the bottem, i have not seen that before. I also did not remember the ejector pin being only half way into the bolt face. It looks like a 556 bolt face is the same way, i should go clean my rifle to refresh my memory. The mark makes more sense to me now.
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Offline Crawl

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Re: Help me understand what happened with my 9mm
« Reply #23 on: June 22, 2020, 04:34:29 PM »
Quick update to let you guys know that CMMG sent me a shipping label so that I can send them my assembled upper, which I have done. There was a note that they may need to replace the barrel. We shall see. Also, I noticed chipping on the bolt face lugs. This gun only has 25 rounds through it, so it's starting to look like a no-go.

Between this and the fact that the Scorpion fires even while on safe (because the plastic flexes too much), I may be selling it all and buying a B&T.

Offline BrandonP

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Re: Help me understand what happened with my 9mm
« Reply #24 on: June 23, 2020, 05:17:26 PM »
That's pretty terrifying. You came close to a whodunit postmortem.

Offline Crawl

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Re: Help me understand what happened with my 9mm
« Reply #25 on: August 20, 2020, 02:42:43 PM »
Update:

CMMG has returned my upper with a new bolt. Their response is that they now offer a newer design, and the extractor was out of spec anyway, so they replaced it. Apparently was not a headspace issue.

You can probably see in some photos how much the tapered rear of the bolt lugs had worn after only a few dozen rounds, so I will be checking that while function and safety testing it.

If there is a new design that works better, then maybe this thing will have some staying power. The magazine seems like a failure point too, but it hasn't been an issue through the first 40 rounds. Maybe after another 2k rounds I can draw conclusions about this gun.

Offline bang bang

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Re: Help me understand what happened with my 9mm
« Reply #26 on: August 20, 2020, 04:13:56 PM »
Did it slam fire?

Is that at stepped 9mm case?  I'm trying to remember what case heads I've seen that have the stepped wall and I think A-USA was one of them.  I'm wondering why this case head didn't separate?  It's hard to tell in the picture.

Obviously it detonated out of battery with part of the case still in the chamber.  Either the bolt is unlocking early or it slam fired before fully chambering.

What buffer and spring was used in the lower?  I contact CMMG and discuss with their tech support folks to get confirmation of your build meets their requirements.

Cheers,
Toby

The bolt is made like an AR15 bolt.  Same style ejector and extractor.

It's a normal 9mm case.  Factory loaded stuff from Expansion Ammunition that has been good for the past 20k rounds or so that I've used it.  I did a plunk test, and the ammo seems to do fine.  There isn't enough unsupported case to explain this much expansion, so it has to be the bolt unlocking early.  I'm using a carbine buffer and spring setup, as CMMG recommends.  It is Armaspec's version of the silent captured spring.  It has given me no trouble in my 223s.  CMMG is still looking into it.

When disassembling it today, the barrel nut came off without a wrench (turned really hard with my bare hands), however, it is a Mega Arms upper...in other words, I had to use a mallet for 10 minutes just to get the barrel removed (which was not surprising because this upper has been this tight with every barrel I've tested with it).  So I don't think the barrel was moving at all in the upper.  Even though the barrel nut was not all the way tight, it certainly couldn't have rotated enough while firing to let the barrel come out of the upper (barrel nut is integrated into the handguard, so any rotation would not have been enough to allow the barrel to move).

I don't see how the powder charge could be the problem.  I've loaded lots of Major power factor ammo.  This powder can't be loaded to those pressures even with a full cartridge.  I'm going to assume that the bolt opened and maybe hung up in the barrel extension somehow delaying the blowback too long.  Still open to brainstorming.


1 of the 1st rules of troubleshooting.... never rule out anything and that includes the user.  Many people put on blinders that never look at the user. 

but its your gun and you will figure it out sooner or later. 

good luck

Offline wanderson

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Re: Help me understand what happened with my 9mm
« Reply #27 on: August 22, 2020, 08:50:49 AM »
This isn’t unusual in a blowback 9mm AR but surprising from a radial delayed setup, that’s the point, not to fire unless it’s locked.
You did right to send it back, a potentially dangerous situation.

Personally, I’d ditch the Armscor, I’ve not been impressed with any Armscor ammo. For me, brass Blazer 115 gr. Is just as cheap and more consistent, and for a few pennies more there’s much better options.

I think if I spent the $$$ for a CMMG I’d buy better ammo like MEN 124 gr. NATO. Which is far from the best, but cheap enough for cheap plinking.

I’d also avoid any aluminum cased ammo, IMHO just too soft for unsupported chambers. I have several blowback carbines that choke on Blazer aluminum but run fine on any brass or steel case.

Offline robert1804

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Re: Help me understand what happened with my 9mm
« Reply #28 on: August 26, 2020, 12:14:28 AM »
That's a classic out of battery fired case. Blowback AR's can be bad about it. A regular AR cannot fire OOB because the bolt rotates into lock-up before the carrier moves forward enough for the firing pin to even reach the primer. AR 9's lack the locking bolt and nothing stops the firing pin if the hammer drops and the cartridge isn't fully chambered. In my case, and many others, these OOB firings are happening when the bolt slams forward hard enough to trip the trigger sear just as the bolt is bouncing back after picking up a new round. My solution was heavier trigger and disconnector springs.

Offline Crawl

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Re: Help me understand what happened with my 9mm
« Reply #29 on: August 26, 2020, 12:16:50 AM »
That's a classic out of battery fired case. Blowback AR's can be bad about it. A regular AR cannot fire OOB because the bolt rotates into lock-up before the carrier moves forward enough for the firing pin to even reach the primer. AR 9's lack the locking bolt and nothing stops the firing pin if the hammer drops and the cartridge isn't fully chambered. In my case, and many others, these OOB firings are happening when the bolt slams forward hard enough to trip the trigger sear just as the bolt is bouncing back after picking up a new round. My solution was heavier trigger and disconnector springs.
I literally posted photos of the bolt...it's a standard AR15 bolt design. It locks into the barrel extension.

 

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