Help PCC brass casing disintegrated

[ichigo]

Hello everybody,

Hope everything is going well during this current situation (Covid-19). As for me, I was in the range today breaking in my newly build PCC (ar9) and at the 38th rounds the brass casing disintegrated inside the upper. Everything is fine, no damage to the firearm and no physical injuries.

Here's how the range time went down, the first 5 rounds (every round is barrel swabbing to remove the manufacturing drilling and rifling tool marking). The succeeding were 5 rounds, check grouping and adjust every run. Before the 26th round (6th run), I did another barrel swabbing. I notice (twice) in the succeeding runs that the hammer is not engaged with the sear. Until the 38th round, the brass casing disintegrated. I want your opinion on identifying what might have cause this malfunction. Does using a single endomag might have cause or contribute to the malfunction.

Thanks in advance

 

[MTRecon]

Can't make out the headstamp on the brass...I'm having a case failure problem with Browning once fired 9mm with loads well inside SAAMI limits...the brass appears weak just above the extractor groove despite fully supported chamber and shooting single shot...

 

[lash]

The brass head stamp is Maxx Tech.

Questions for the OP:

• What exactly does it mean that the hammer was not engaged with the sear? Would it not fire?
• I cannot tell what the second picture shows. What is that?

As to the brass, it looks like that case was not supported when it was fired. Almost as if it wasn’t completely in battery when the firing pin struck the primer.

 

[roostercogburn98]

What system is this. Straight blowback, radial blowback? I agree with @lash on OOB discharge. That case looks like it had zero support when it detonated. 9mm ARs can be prone to bolt bounce if the recoil system is not set up correctly. Details will help this get diagnosed.

 

[TxWelder35]

Were all the rounds blowing up the brass or just the one?

 

[ichigo]

q1: Questions for the OP:

• What exactly does it mean that the hammer was not engaged with the sear? Would it not fire? ----> will not fire / can't squeeze the trigger, i have to clear the round in the chamber and it will work again
• I cannot tell what the second picture shows. What is that? -----> the second picture is the casing still held by the extractor / bcg

q2) What system is this. Straight blowback, radial blowback?
a2) It's an ar9 and believe it's a direct blowback system

build setup:
aero lower
ar-stoner upper/handrail
kvp 8.5" barrel
kvp bcg
anderson LPK (bundle with SBA4, FCG is silver in color)
buffer spring is heavy 308
kvp 7.5oz buffer
kvp muzzle brake

q3) Were all the rounds blowing up the brass or just the one?
a3) just one round (38th round) and immediately stop. Tried to remove the BCG in the range but since the head casing is still held by the bcg, the bcg was a few millimeter inside the buffer tube and also not sure if disassembling the firearm is allowed in the range. Until I reached home that I reliaze that if I take both the front/rear takedown pin and have the upper slide forward, then I was able to remove the bcg.