9mm case blown apart,Why?

SCARCQB · Feb 20, 2020

I had a similar event with a home assembled AR 9 pistol.

I made the error of using a standard Weight AR buffer. Since these are direct blowback weapons. I may have had timing issues and an out of battery discharge. I switched to a stainless steel 10 oz pistol buffer( longer and heavier) It’s been reliable since.

When I had the standard buffer in. ( 3.6 oz) the fired cases were extremely bulged at the base.

Doobie · Feb 20, 2020

I hate when people post stuff like this asking for a cause/reason, but leave out very important specifics such as make and model of firearm. There’s dozens of reasons that this happened, but knowing what specific firearm it happened in can eliminate a lot of guessing and assuming. Rant over.

Doobie · Feb 20, 2020

Here’s an example of how it should be written:
These two blown primers happened on the same day within the same hour. The ammo is factory Speer 124 TMJ 9mm. One blown primer happened in a completely stock Glock 17 Gen 4 9mm and the other in a completely stock Colt 9mm SMG. The SMG was being fired in semi auto mode when it occurred. Other than lot number of ammo, temperature, and humidity, I believe I gave enough detailed info. Everyone should do the same .

John from MD · Feb 20, 2020

fred55 said:
Wouldn’t an out of battery show a pierced primer? Did the primer strike look different from the rest? fred55

No, there seems to be no relation to the two. The OOB detonations I have seen had normal primers.

John from MD · Feb 20, 2020

Doobie said:
Here’s an example:
These two blown primers happened on the same day within the same hour. The ammo is factory Speer 124 TMJ 9mm. One blown primer happened in a completely stock Glock 17 Gen 4 9mm and the other in a completely stock Colt 9mm SMG. The SMG was being fired in semi auto mode when it occurred. Other than lot number of ammo, temperature, and humidity, I believe I gave enough detailed info. Everyone should do the same .

They are pretty unique blown primers. I have seen pierced primers and blown primers where the pocket was empty but I have never seen damage such as yours.

Doobie · Feb 20, 2020

John from MD said:
They are pretty unique blown primers. I have seen pierced primers and blown primers where the pocket was empty but I have never seen damage such as yours.

Believe it or not those are the first blown primers I have ever encountered in my decades of shooting. What is bizarre is that they happened within 20 minutes of each other to different guys standing side by side on the firing line.

John from MD · Feb 20, 2020

I have seen plenty as I experimented in the sixties with surplus military powder that there was no data for. However, yours are really different. It almost looks like the primer pockets expanded but the primers didn't come out but somehow one edge got rolled over. The REALLy weird part is that they were fired in two different guns. It would be worth sending them to Speer to see what they have to say about it.

atblis · Feb 20, 2020

RFD said:
factory ammo

Are you certain? Are they "remanufactured" rounds?

Doobie · Feb 20, 2020

John from MD said:
I have seen plenty as I experimented in the sixties with surplus military powder that there was no data for. However, yours are really different. It almost looks like the primer pockets expanded but the primers didn't come out but somehow one edge got rolled over. The REALLy weird part is that they were fired in two different guns. It would be worth sending them to Speer to see what they have to say about it.

The rounds didn’t ignite either. I don’t have the lot number. The ammo was stored in factory boxes in a climate controlled armory.

Melnic · Feb 20, 2020

wait, I"m no gunsmith but isnt the base of the case not supported all the way around on most rimless guns? I know a friend's sig 40cal pistol for example has a bunch of the base of the case unsupported and thus the case has a certain amount of thickness to it. Couldn't this be a failure of the case in the photo by the OP?
WHo is to say this was not simply a failure in the case?

John from MD · Feb 20, 2020

Melnic said:
wait, I"m no gunsmith but isnt the base of the case not supported all the way around on most rimless guns? I know a friend's sig 40cal pistol for example has a bunch of the base of the case unsupported and thus the case has a certain amount of thickness to it. Couldn't this be a failure of the case in the photo by the OP?
WHo is to say this was not simply a failure in the case?

With the exception of Glock, most guns have chambers that fully support the case head. If a case if fully chambered, the only way it comes apart is excessive pressure. If the case is loaded correctly and fully chambered, cases don't fail. The failure mode of my cases is usually a crack at the shoulder or mouth.

Glock chambers don't like repeated reloads as the case does get stressed just in front of the case head. When I was competing with Glocks, I always dropped a Barsto barrel into it.

kalister1 · Feb 20, 2020

The FC cases look like a two piece case, Like the primer end was pressed into the rest of the case? Never seen anything like that.

The two Blown primers looks like the were installed side ways?

1time · Feb 20, 2020

Mine looked like that. Was bump firing a 9mm AR and it lit off while oob. That was before the bump stocks and why I cringed when I saw bump stocks on blowback guns.

GuitarmanNick · Feb 22, 2020

Just experienced my first OOB on a home built Polymer80, 9mm. It did not survive! An aluminum lower would not have been damaged IMO.

This gun has been perfectly reliable until today but I decided to fire the last mag at a faster than normal pace. Surprised the crap outta me.

Probably bolt bounce from the looks of everything. Mag well split apart at the front corner and the PMag body housing the Endomag also split apart. The Endomag insert had no damage.

Doctor_M · Feb 22, 2020

Glad you didn't get hurt... wow.

RuralRifleGuy · Feb 23, 2020

GuitarmanNick said:
Just experienced my first OOB on a home built Polymer80, 9mm. It did not survive! An aluminum lower would not have been damaged IMO.

This gun has been perfectly reliable until today but I decided to fire the last mag at a faster than normal pace. Surprised the crap outta me.

Probably bolt bounce from the looks of everything. Mag well split apart at the front corner and the PMag body housing the Endomag also split apart. The Endomag insert had no damage.

Well damn, you commented on my post about the Polymer80 lower last week about no issues with it. Now I’m concerned my question jinxed you. Glad you’re ok, mine will become a .22lr after reading this.

Pinecone · Feb 23, 2020

OOB would not normally have a pierced primer, as the case ruptures at lower than full pressure. So unless primer would have come out with a fully in battery firing, it will not come out in OOB.

I agree that it looks like OOB.

And OOB should NOT occur because you fired to fast.

Hammer should not fall until the slide/bolt is fully in battery. Otherwise you have some defective parts or incorrect dimensions.

1time · Feb 23, 2020

Pinecone said:
OOB would not normally have a pierced primer, as the case ruptures at lower than full pressure. So unless primer would have come out with a fully in battery firing, it will not come out in OOB.

I agree that it looks like OOB.

And OOB should NOT occur because you fired to fast.

Hammer should not fall until the slide/bolt is fully in battery. Otherwise you have some defective parts or incorrect dimensions.

What parts would prevent the hammer from falling prior to the bolt being completely closed?

erwos · Feb 23, 2020

1time said:
What parts would prevent the hammer from falling prior to the bolt being completely closed?

Auto sear.

Sent from my Pixel 4 using Tapatalk

1time · Feb 23, 2020

erwos said:
Auto sear.

Sent from my Pixel 4 using Tapatalk

Lol. That’s all I could come up with, but admittedly I am not a gunsmith.

9mm case blown apart,Why?

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