9mm case failure

[nofishbob]

The above discussion about the definition of an "unsupported chamber" has left me confused.

I thought that when people discussed case failures in the 6 o'clock position on the case head and blamed an "unsupported chamber" they were referring to the crescent-shaped area of case exposed at the feed ramp.

Now are we saying that an "unsupported chamber" is one that is oversize or loose?

Thanks!

Bob

 

[The Bushmaster]

To my knowledge the dreaded unsupported chamber eludes to the fact that a cartridge fired in one will have a belly swelling at the 6:00 area upon extraction and ejection. And if reloaded and being chambered in that same position again can and have ruptured at the belly just forward of the case head.

I have picked up many cases that have that expantion just forward of the case head and have discarded them. I have seen several cases blown at that same place and have figured that they came from an unsupported chamber as discribed by Galil5.56 in his photos...

Neither my Colt 1911, Kimber or Firestar have unsupported chambers. But my BTA-90 is unsupported and balloons the cases at the 6:00 position just forward of the case head.

 

[Galil5.56]

To confuse the matter further, I have read some folks define unsupported chamber only if the case web and side wall juncture is above the chamber... i.e. my M9 picture would fall under this assertion, but my P220 as shown would not. Since this case wall web junction can vary widely even within the same brand of brass, I will continue to define "unsupported chamber" as I have, and any chamber diameter above the SAAMI spec shown below as oversize, but not necessarily bad.

 

[Shibumi]

"Greetings,

Shibumi, how old is your Glock? I read at a few places that Glock redesigned their barrel a few years ago for the .40 and .45."


Mine was born in May 2008. I'm not sure of the years of demarcation, but it is a Generation III designation. Previous iterations had more open area at the feed ramp and were capable of ejecting "pregnant guppy" brass in the right circumstances...

 

[amlevin]

I had a near identical failure in my CZ75 SP-01 Tactical a couple of months ago. Close examination of the case showed a circular crack on the inside where the head joined the sidewall of the case. The case was a "range pickup" (like the other 8,000 or so in my shop) and it just got tired. I load thousands of rounds (all 124gr) using 6.5 gr Power Pistol and for my "hotter" loads 6.6 gr of VV-3N37. Out of these thousands of rounds I have had only ONE failure like this. Usually the case just splits from the mouth down and I catch it on inspection after cleaning.

Keep your loads below max and wear safety glasses. Stuff like this Can also happen with new ammo. Nothing's perfect.

 

[funnelcake]

I've been using 125gr. cast RN over 4.2gr. HP-38 at ~1.128 & it's done very well in everything from a Luger to a Cougar.

Funnel