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possible near Kaboom cause(s) #16538061 10/17/21
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m1rifleman Offline OP
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Grandson was shooting a 9mm AR rapid fire when the last round (15th?) blew the mag out. An inspection showed no external damage, but when the bolt was removed what was left of the case head was still stuck to the bolt face. Looking at spent brass showed several with bulges near the case head. We called it a day with that rifle and later ran the remaining loaded rounds in a Wilson case gauge. lots would not seat below flush. FWIW this is remanufactured ammo from a reputable firm.will PM anyone who wants to know the name. This obviously is bad juju, but can anyone think of another cause for the bulges or separated case?Barrel was checked with Go/No go and passed.


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Re: possible near Kaboom cause(s) [Re: m1rifleman] #16538309 10/17/21
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dan_oz Offline
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Bulges near the case head suggests either a chamber which doesn't support the case head, such as is the case with some pistols with heavily ramped chambers, or firing out of battery. Even then at usual 9 mm pressures with sound brass you would not expect cases to let go.

It may be that the brass was bulged in the gun it was first fired in. That much is suggested by the fact that it won't seat in your gauge. The bulging and whatever resizing may have cold worked it a bit. If it isn't seating properly in your chamber and your rifle is still firing it (ie out of battery) that may be what is caused the failure, especially if the stuff is running at high pressures.

Might be a case for tearing it down to save the projectiles and binning the brass.

Re: possible near Kaboom cause(s) [Re: m1rifleman] #16538328 10/17/21
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Re: possible near Kaboom cause(s) [Re: m1rifleman] #16538378 10/17/21
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Every 9mm AR I've ever seen used a bolt that was non-rotating/locking, but still essentially fully surrounded the case head. If fired cases are notably bulged relative to unfired ones, then you have a timing problem. Either pressure is still high after the bolt moves back, or the gun is firing before it goes into battery.

Since the rounds gauge oversized to start, I would start by removing the bolt and checking if 100% of rounds will "plunk" in the chamber. If not, there's a good chance that you've got rounds firing out of battery. if they all drop in with just gravity, it's likely it's unlocking too soon.

Re: possible near Kaboom cause(s) [Re: m1rifleman] #16538434 10/17/21
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I just love this crap. ALMOST NOTHING can fire OOB. The design makes it impossible. I have never figured out where guys come up with this BS. the bulgingh cases are almost certainly from the bolt opening prematurely, before chamber pressures can be lessened. I have seen a S&W Mod 38 wadcutter pistol do this. We noticed a bulge in the cases as we fired it. No issues other than the bulge, but it wasn't staying locked long enough. No idea as to the cause in this case. My BIL also has a Glock that was a police turn-n that was also doing this. I looked at the cases. Same bulge. I noticed the slide was very easy to open and thought it may be the mainspring. We took it apart and the spring was bent a bit. He replaced it and good to go. I am betting there is something allowing the slide to open prematurely.


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Re: possible near Kaboom cause(s) [Re: m1rifleman] #16538522 10/17/21
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dan_oz Offline
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Originally Posted by JeffP40
ALMOST NOTHING can fire OOB. The design makes it impossible.


The typical 9 mm AR upper is an example of a design in which out of battery firing actually can occur, due to the design um, not making it impossible. Using a S&W Mod 38, whatever that is (unless you meant a S&W Model 52, rather than Model 38) and a locked-breech pistol as examples to support your argument suggests that you may not realise that these are not applicable to a simple blowback design.

Re: possible near Kaboom cause(s) [Re: m1rifleman] #16538524 10/17/21
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bowmanh Offline
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If the remaining unfired rounds include a bunch of oversized cases then it sounds like an ammo problem, although there could be multiple factors at work. I wouldn't want to shoot any more of that ammunition, but I would also look carefully at the rifle. I've shot a lot of 9mm AR rounds and have never seen any such issues. I did have a case separation recently during a match (my first, didn't hurt the rifle or me) that likely separated at the case cannelure on some range brass. I'm now being careful about using range brass with a cannelure in a blowback AR.

Re: possible near Kaboom cause(s) [Re: JeffP40] #16538558 10/17/21
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Originally Posted by JeffP40
I just love this crap. ALMOST NOTHING can fire OOB.


On the the 5.56 bolt the firing pin cannot protrude until the bolt cams and moves rearward in the carrier. The pin is too short. But in the 9mm AR without a rotating bolt, what provides the same protection? I don't see anything, but then again I don't have OP's bolt in hand either.

Re: possible near Kaboom cause(s) [Re: dan_oz] #16538561 10/17/21
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Originally Posted by dan_oz
Originally Posted by JeffP40
ALMOST NOTHING can fire OOB. The design makes it impossible.


The typical 9 mm AR upper is an example of a design in which out of battery firing actually can occur, due to the design um, not making it impossible.

Yeah, I'm not seeing it either.

I was mentioning on a previous thread there are lots of people who shoot guns, own guns, and don't know [bleep] about guns. I think we have an example of that here.

Re: possible near Kaboom cause(s) [Re: m1rifleman] #16541510 10/18/21
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autoshopteacher Offline
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What brand cases were involved? I experienced problems with cases bulging and a few burst in a Beretta pistol. Faulty cases were PMC. A noted Gunwriter (not on this forum) witnessed this activity and said the PMC cases had a thinner web at the base of the case and were weaker as a result. I got home and pulled some bullets in several case brands, cut the brass longitudinally and and the PMC were thinner where the base transitioned into the case wall. The failures were aggravated by the base not being fully supported when chambered. I ended pulling nearly 1000 bullets and disposing of the PMC brass. Years ago and no problems since.
These were reloads with a nearly max powder charge as listed in my loading manual.

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Re: possible near Kaboom cause(s) [Re: autoshopteacher] #16541827 10/19/21
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m1rifleman Offline OP
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teacher, welcome to the forum. someone will be along shortly to piss on your response instead of offering constructive criticism. some on here just feel compelled.....


Those who would disrespect our flag have never been handed a folded one.

Men never do evil so completely and cheerfully as when they do it from religious conviction.

When in the Course of human events......

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