Failure Diagnosis

Slosolo2

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A friend has a 300blk pistol that isn't cycling more than 2 rounds before failure.
Not sure what length barrel, more than 8 less than 10. H2 buffer. No suppressor. A2 flash hider.

Went to the range today and did some testing. Started by firing 5 sub and 5 supersonic from my gun to confirm function then fired the suspect gun to confirm it still had a problem. It failed to eject on first round mangling a brand new casing.
We then swapped uppers between the lowers.
Fired 5 of each type through the suspect gun lower with my upper without malfunction.
Suspect upper on my lower didn't go well.
Supersonic
First round ford and cycled correctly
2nd round fired but 3rd round failed to feed as in it was still in the magazine so third trigger pull was click no bang.

Subsonic
First round failed to eject and 2nd round was stripped from mag. Both the spent and unfired round tried to enter the chamber. They were jammed but easily dislodged.

Took it home and squirted wd40 through the gas tube whole looking down the barrel. First squirt nothing came out but the second squirt did and it seemed to push a clump of something out. Note this gun has less than 30 rounds through it total. Upper was purchased fully assembled from the oem.
Ran a patch but didn't get a clump just the usual carbon.
Pulled the handguard.
Gas block looked to be properly aligned.
Pulled the gas block but not flag hider.
Barrel hole is unobstructed.
Didn't un-pin the tube.
Checked the bolt for proper function. It stands up without collapsing but is easy to push down. I did suggest he disassemble and check to ensure it's built correctly and lube it generously.

My thoughts are that it needs more gas or less buffer.
My fear is the upper is not machined square and binding the bolt. Is that even possible?

Anyone have suggestions or insights on further testing or what might be wrong with it?
 
Not sure on 300blk, but if it were 5.56 I’d suggest a heavier buffer in that short of a barrel. Reason being that a lighter buffer with that much gas, even with a short dwell time, would cause the buffer to “outrun” the magazine’s ability feed a cartridge.

Good luck. I’m out of my league trying to diagnose 300.
 
Either overgassed or buffer. Take your pick. I'd say overgassed.
 
Not sure on 300blk, but if it were 5.56 I’d suggest a heavier buffer in that short of a barrel. Reason being that a lighter buffer with that much gas, even with a short dwell time, would cause the buffer to “outrun” the magazine’s ability feed a cartridge.

Good luck. I’m out of my league trying to diagnose 300.
Thanks for the input.

Funny thing is my barrel is 9.5" with an H2 buffer and runs no problems.
I built mine in my messy garage from a hodgepodge of parts.
 
swap out bolt and lower with known working ones. if it doesnt run you have isolated the problem to the gas system. if it runs, could be the bolt or buffer. also, check gas key on suspect bolt. loose gas keys do strange things. key/tube alignment also a possibility.
 
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swap out bolt and lower with known working ones. if it doesnt run you have isolated the problem to the gas system. if it runs, could be the bolt or buffer. also, check gas key on suspect bolt. loose gas keys do strange things. key/tube alignment also a possibility.
Thanks
We did swap the upper onto my lower with no improvement. Problem is we both have H2 buffers so that leaves some questions.
We thought after leaving the range we should have swapped bolts just to check off more possible issues.
Fwiw he's running BCM bolt and carrier. Mine is a palmetto I picked up cheap.
 
Thanks for the input.

Funny thing is my barrel is 9.5" with an H2 buffer and runs no problems.
I built mine in my messy garage from a hodgepodge of parts.

That’s the best way to build them. Haha

Inconsistencies in the gas port from barrel to barrel could lead to an over- or under-gassed issue. I would try a heavier builder if possible to see if it improves, and also try a standard buffer.
 
Fwiw he's running BCM bolt and carrier. Mine is a palmetto I picked up cheap.
bcm bolts dont have gas key 'problems'

is the block adjustable?

I would try a heavier builder if possible to see if it improves, and also try a standard buffer.
agree 100% However, I would go lower first, based only on how you described the failures.

also, what buffer spring ?

As mentioned above, make sure you are using a magazine with a fresh spring in it to eliminate any other potential issues.
 
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I was thinking undergassed because it worked a ltitle better with the supersonics. Can you explain your diagnosis? I'm no AR mechanic but willing to learn.

Reason - you stated the case was left in the upper. The extractor claw is being yanked off the rim. A couple things mainly cause that - wrong buffer and/or overgassed. Depending on the burn rate of the powder you're using coupled with gas port location, peak pressure could be hitting the port just after the bullet passes leading to over pressure on bolt/ carrier group overcoming the extractor. Had the same issue on my carbine but in my case, about twice per magazine it happened. I increased the extractor pressure and added an adjustable gas block, pesto, it hasn't bobbled since.
 
Did you swap the BCG from the offending build into yours and vice versa? Trying to narrow down the problem as much as possible
 
Reason - you stated the case was left in the upper. The extractor claw is being yanked off the rim. A couple things mainly cause that - wrong buffer and/or overgassed. Depending on the burn rate of the powder you're using coupled with gas port location, peak pressure could be hitting the port just after the bullet passes leading to over pressure on bolt/ carrier group overcoming the extractor. Had the same issue on my carbine but in my case, about twice per magazine it happened. I increased the extractor pressure and added an adjustable gas block, pesto, it hasn't bobbled since.
I should have said I bought Remington Green and white box to do the testing, a box of 120gr and a box of 220gr. I had shot these in my gun before and it was what Cabelas had from the same brand with the same basic design in both classes.
Both weights worked in my upper on either lower.


Did you swap the BCG from the offending build into yours and vice versa? Trying to narrow down the problem as much as possible
We didn't think to do that until we were driving away from the range. He will try again and swap to a bolt from one of his other guns.


bcm bolts dont have gas key 'problems'
is the block adjustable?
agree 100% However, I would go lower first, based only on how you described the failures.
also, what buffer spring ?
As mentioned above, make sure you are using a magazine with a fresh spring in it to eliminate any other potential issues.
I wouldn't assume BCM has gas key problems but all manufacturers have defects occasionally.
Unfortunately the gas block is fixed not adjustable.
I'm not sure what buffer spring he is using, mine is a carbine buffer spring.
We also used the same magazine for all testing. This was one of my Pmags but he's had the same problems using Lancer mags as well. All the mags are relatively new.
 
Reason - you stated the case was left in the upper. The extractor claw is being yanked off the rim. A couple things mainly cause that - wrong buffer and/or overgassed.

Could also be rough or dirty chamber maybe.
Be worth looking at.
 
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