Help Diagnose 300blk Build Over-Gassed?

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Keeebs

Keeebs

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New 300blk 8" barrel pistol build. Couple 1-200 rounds. Before I start throwing money at this, I was hoping for some confirmation. I believe the gun is over-gassed and the bolt is cycling to fast. I've taken the donut from the extractor, which caused the gun failure to eject. My next thought is new BCG and buffer/spring. Hoping someone can confirm one of these 2 parts are what I need or am I about to spend money when the issue could be elsewhere?

Observations
The brass has snake bites on neck.
Rim is chewed up way more than any of my factory 556 rifles.
Primers looks fine. 110-150gr is what I have, symptoms on all brass.
Mouth is "D" which seems normal from short case hitting deflector.
Deflector has brass marks pretty low towards the BCG, I would of expected these further out.
Buffer has BCG marks which none of my other 556 have (possible cheap PSA buffer & spring?)
Buffer is chewed up from something.


Upper is all Aero.
https://www.aeroprecisionusa.com/300-blackout-8-cmv-barrel
https://www.aeroprecisionusa.com/750-lo-pro-gas-block
https://www.aeroprecisionusa.com/gas-tube-melonite
https://www.aeroprecisionusa.com/ar15-556-bolt-carrier-group

Lower is PSA lower kit.
Buffer weight is 3oz.

My Images
https://ibb.co/gMMLv61
https://ibb.co/GWTHGnB
https://ibb.co/wdSDc9Y
https://ibb.co/FgwtkVZ
https://ibb.co/1zWZT99
https://ibb.co/yQ2skfm
 
I did not build my 300, but bought the upper (mostly Seekins). I had cycle problems. Since I had an adjustable gas block, I was able to get running (though it took some tinkering). Have you considered an adjustable gas block?

https://www.aeroprecisionusa.com/adjustable-gas-block-750-low-profile

Another thread: https://www.thehighroad.org/index.php?threads/300bo-upper-not-cycling-properly.897385/#post-12109636
My thread: https://www.thehighroad.org/index.p...ar-gas-block-for-300-bo.880592/#post-11764535
 
If you have another buffer (H2) that you can rob from another gun to try would be my first try. Adjustable gas block will be the most beneficial. I prefer the JPI adj gas blocks, clamp-on type. Now if your barrel is all ready drilled for the dimples the std work fine. I get less leakage from the clamp-on.
 
As you said, your gun is over gassed and the bolt is cycling too fast. It’s also cycling too SOON, trying to pull the round at too high chamber pressure.

1. Fix the too SOON first.
a. Buy and install an adjustable gas block. Or drill and tap your fixed gas block with a screw that can fully block the gas block hole between the barrel and gas tube. Drill/tap is what I do because it costs next to nothing.
b. Polish your chamber including the neck area

2. Fix the too FAST second.
a. Use you gas block adjustment to get 10 empty cases to land in a tight clump a couple feet away
b. Adjust buffer tube weight so all your ammo functions 100%
c. Clean gun to make sure always functions, clean to dirty

You’ll read buffer weight can delay the impulse. But my experience is that you can’t get there on a severely over gassed gun. I just finished a 7.5” and I did the steps described above. Went from 50% to 100% reliable, factory loads, hand loads, clean, dirty, supported, unsupported, don’t matter :)

index.php
 
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If it's destroying brass get some tungsten weights to put inside your buffer. You should be able to get 3 weights for about $30.
That's the cheapest, easiest to do.
Then there's pig tail gas tubes, normally they're easiest to find for pistol gas systems, price is up to $50.
Alright I looked at the pics, yeah hellishly over gassed.
You might need some combination of tungsten weights in your buffer and a pigtail.
Extractor might be beat up from trying to rip those cases out of battery while the cases is under pressure. Do you have another AR you can swap that extractor over on?
 
Nearly every carbine sold is over-gassed with a cheap, too light carbine buffer yet they still work. Carbines are meant to rip brass out because the military doesn’t stop to pick it up. So…it’s not ejecting, leaving brass in the chamber…you have an extractor problem.

Get it running first, address peripheral issues later.
 
We're not the military. A good portion of us want to reload our brass and want our guns to last as long as possible. We don't have to have a gun and ammo that will work from Greenland to Tanzania and everywhere in between.
 
N555 said:
We're not the military. A good portion of us want to reload our brass and want our guns to last as long as possible. We don't have to have a gun and ammo that will work from Greenland to Tanzania and everywhere in between.
Click to expand...


I’m a reloader as well, but my point is not to confuse the issue with “maybe try this” or “it could be that so change the other”. In this case, the empty cartridge is staying inside the rifle where it shouldn’t be. Went from taking bites to removal of o-ring to losing control of the case.

So we fix that before worrying about marks on brass. A heavier buffer or AGB can tune that out later if needed but first we want to avoid:


Keeebs said:
My next thought is new BCG and buffer/spring. Hoping someone can confirm one of these 2 parts are what I need or am I about to spend money when the issue could be elsewhere?
Click to expand...

because that’s $100 or more needlessly spent that won’t fix extraction or brass (unless the new BCG has the upgraded spring, which still leaves the brass chewed up). I wasn’t meaning to be obtuse earlier about the peripheral issue, just starting from reliable function before fine-tuning.
 
I figured it was possibly trying to rip the case out of battery while it's still under pressure.
As you can see a pistol length gas system is going to feed the gas system near peak chamber pressure. Say a 300bo is only running say 40,000psi at 3 inches, that's still screaming.
gassystem.png
 

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I went through the same problem with a AR in 22 Nosler. Quieted it down quite a bit with Tungsten buffers.
 
N555 said:
I figured it was possibly trying to rip the case out of battery while it's still under pressure.
As you can see a pistol length gas system is going to feed the gas system near peak chamber pressure. Say a 300bo is only running say 40,000psi at 3 inches, that's still screaming.
View attachment 1087387
Click to expand...

That is correct, opening too soon, but the extractor has a sole purpose independent of anything else: control that casing! So getting the OP back to functioning then addressing the (likely) carbine buffer replacement with an H2 should make everything the way it ought to be (for a brass saver).
 
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