Help needed, Failed No-Go .300 AAC, pictures of brass.

New build.

Barrel: 8" CMMG .300 blackout barrel
BCG: AIM surplus With an RCA adjustable gas key set to lock back on empty mag with subs no suppressor.
Ammo: Magtech 123grn

Current round count is about 200-225 mixed subs and supers.

So last week i put 150 rounds through the gun. I got home and started resizing brass, i had 4 out of the 150 that jammed in the sizing die and formed a ledge toward the bottom of the brass i thought maybe it was the die or possibly not enough case lube. I took a note and set them a side. Picture below.

https://i.imgur.com/un3mwr8.jpg

Today i planned on running another 150 rounds of factory loads before i started shooting my reloads. I kept the oddity in mind while shooting today and kept a close eye on the brass. Heres an odd pattern i noticed, its not every piece its like 25% of the time.

https://i.imgur.com/IpYkOL0.jpg

The observant ones out there may have noticed the missing primer. This is when i stopped shooting and picked up all my brass. There was only that one that lost the primer. Upon closer inspection i found a few that had significant deformation at the ejector location and extractor marks on the rim. In some pieces the brass appears stressed as if it has stretched. I cherry picked the worst ones and took pictures.

https://i.imgur.com/NOaiuQw.jpg
https://i.imgur.com/NQ1D1tc.jpg
https://i.imgur.com/DFP68R5.jpg
https://i.imgur.com/utu3dMS.jpg

Gunsmith had a set of 300blackout Go / No-Go gauges. I had a spare PSA Premium BCG. We removed the ejectors and checked with the AIM BCG and it closed on the No-Go. We checked with the PSA bolt and it did not close on the No-Go. He then grabbed a known good .300blk barrel off the shelf and the AIM bolt did NOT rotate on the no go. The gunsmith only did this with his fingers and did not use a punch like i have seen some do to assit in rotating the bolt. I wish i would have had him double check but im home now.

So.... Is it the bolt? Is it the barrel? Is it the ammo? Id like to think its just the bolt as the PSA bolt did not close on the No-go. However with the AIM bolt also not closing on No-Go in a different barrel that makes me wonder. The brass shows signs of problems that i haven't seen before and have no experience diagnosing. Opinions needed!

Gunsmith said just run the PSA bolt...

https://i.imgur.com/JzgiUnS.jpg

Tolerance stacking between the bolt, barrel extension, and chamber. It happens. Use your PSA bolt and yse the AIM bolt for something else.

Nobody but your gunsmith can really say. There is no standard for a NoGo, some are made for cutting bolt gun chambers and are only a few thou over the Go. Without the numbers I can't say what that gauge means.

I can say dropped primers are a warning sign of pressure, not headspace.

Agree with Todd. I'm seeing signs of high pressure. Ejector flow, a popped primer, that belted magnum look at the base. At a minimum, run the PSA bolt and continue to check for those signs to eliminate the ammo.

Okay guys here is what i have figured out.

1) Magtech 123grn seems to be loaded hot, as well as inconsistently. About 1 out of every 5 shots fired will exhibit signs of over pressure, ejector smear, firing pin channel extrusion. This is the only ammo i have that does this, the magtech 115gr, and fiocchi 150gr both shot without any signs of over pressure. I intend on testing more ammo.

2) The lightweight AIM bolt carrier, with a 0.8oz buffer, tuned with an RCA gas key to lock back on empty, with subs, unsuppressed, Is far too overgassed with supers and a suppressor. Given the system is overgassed, running a hot round like the magtech 123gr mentioned above would result in sever extractor marks and in some cases stretching the case or unlocking the bolt while case pressures are still extremely high allowing the case to grow rearward. Im surprised i didnt experience any failures to extract.

Simply shooting other ammo alleviated all symptoms. Shooting with a standard weight m16 BCG and h1 buffer still showed the occasional ejector smear with the 123gr magtech but extractor marks and case stretching seemed to diminish.

Conclusion: This batch of 123gr magtech exhibits symptoms of excessive pressure. A lightweight carrier and stripped buffer with an adjustable gas key tuned to run subs unsuppressed does not do well with suppressed supers. My guess is that even if the gas key is closed off substantially, gas tube pressures are high enough to push the lightweight carrier and buffer rearward to unlock the bolt and start the extraction process. Running a lightweight carrier and buffer settup suppressed probably requires an adjustable gas block for reliable function.

Thank you for the input!