Assistance with under-gassed ar15 build?

[CharlesAFerg]

After about a year of gathering parts, I finally built this rifle and test fired it. Unfortunately, it seems to be under-gassed. The rifle will fire, but no more than 5 times in a row, will often not strip a round off the mag, and it will not lock back on empty. It is ejecting spent cartridges. My biggest concern is how to validate whether or not it was the gas tube installation or if the pinned gas block is the culprit, in which case I would have to send it back to Centurion arms.

Relevant build info:
Aero upper, lower, and CBG.
16" Centurion arms mk12 mod H barrel w/ pinned gas block (was assembled) but I installed the gas tube as I built it.
Geissele Super 42 w/ H1 buffer

Perhaps I can get some feedback on how to properly troubleshoot it? Anything helps!

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[Joe13]

You have another AR you can swap parts on?

 

[Member 34319]

Missing has length info. Buffer and spring might be problem. Go lighter. I'd still having issues with lighter spring and buffer, then report back.

 

[Cerberus Group]

Might have an out of spec gas tube, or gas key on the bcg.

Also, the barrel/pinned gas block may have out of spec ports, out of alignment etc

 

[CharlesAFerg]

Missing has length info. Buffer and spring might be problem. Go lighter. I'd still having issues with lighter spring and buffer, then report back.

Mid length, mod Holland barrel. Seems odd that a super 42 w/ H1 would cause problem? Seems like that'd make me the outlier, which makes me suspect it's a gas system problem. I'll switch it for a standard spring & buffer, regardless.

 

[RX-79G]

It is always worthwhile to ask Centurion Arms. They might have information about other reports of undersized ports or something.

 

[msgriff]

Mid length, mod Holland barrel. Seems odd that a super 42 w/ H1 would cause problem? Seems like that'd make me the outlier, which makes me suspect it's a gas system problem. I'll switch it for a standard spring & buffer, regardless.

It's definitely worth trying a standard spring and buffer. Were you using the Geissele buffer with the 42?

 

[edzz]

Try different ammo and run it wet. I've had ARs that didn't want to run dryish till they had been shot a little.

 

[Caveman Jim]

Gas block misalignment from factory, gas tube misalignment or gas key.

 

[zomie]

May want to check the gas port size and go up a bit. There are gas port charts out there for the ar15 you can verify against.

 

[Member 34319]

Mid length, mod Holland barrel. Seems odd that a super 42 w/ H1 would cause problem? Seems like that'd make me the outlier, which makes me suspect it's a gas system problem. I'll switch it for a standard spring & buffer, regardless.

It might be a stretch saying they are the cause. It could be a handful of things listed above. The barrel could have been misaligned when drilling the holes for the gas block and your gas port is partially covered. Underpowered ammunition is also a typical culprit. Lubrication can be a culprit. The list goes on.

Whenever I put an AR together or buy a factory one. I always start with the lightest buffer and spring possible. Even if it came from the factory with heavier. If it runs hot, I'll add weight to the recoil system till it starts short stroking. Then step back a weight or two till it functions with 4-5 different ammunitions 100%. For instance. I open a factory Springfield AR and it shipped with an H buffer. After doing the usually shoot and swap, it ended up with a H2 by the time I finished and it shoots like butter. I've had carbine length PSA's that barely ran cheap ammo with the absolute lightest of buffer set ups.

Regardless of what the barrel manufacture recommends, or what the internet says, I usually tell folks to start at the beginning. If it don't work with a carbine buffer and spring, then I tell folks to start measuring things. That's usually if the most obvious culprits are isolated first. Like ammo and giving it some lubricant.

The spring and buffer can help reduce recoil, but they are so crucial to proper operation of the action that I would rather the gun recoil more due to lighter buffer and springs. Then solve recoil some other way like a muzzle brake.

 

[NobullSKS]

At which location/position on "clock" is the shell ejecting to…..1…2…..3…4..?

 

[powermad]

Those barrels have a notch on the barrel and a tab on the gas block for alignment.
The gas port size is listed at .076", perfect for a 16" middy and should run well with a blue spring and H buffer or an A5H2 buffer and green spring.

A new pin is probably 50¢ from Tacoma Screw, drive it out, scoot the GB back a bit and measure the port.
You'll be able to see if the notch is directly behind the gas port also and not off to one side or the other.
They are a solid company, I'd be surprised if something is out of spec but anything is possible.
At any rate I'm certain they will take care of it if something is goofed up.

 

[powermad]

Before looking at the gas port I would be looking at everything else down stream first.
Like the BCG, it's not unheard of to see a botched gas key install or just out of spec all over.
I'd swap in a known good BCG and see what happens.

 

[Unicykle]

After about a year of gathering parts, I finally built this rifle and test fired it. Unfortunately, it seems to be under-gassed. The rifle will fire, but no more than 5 times in a row, will often not strip a round off the mag, and it will not lock back on empty. It is ejecting spent cartridges. My biggest concern is how to validate whether or not it was the gas tube installation or if the pinned gas block is the culprit, in which case I would have to send it back to Centurion arms.

Relevant build info:
Aero upper, lower, and CBG.
16" Centurion arms mk12 mod H barrel w/ pinned gas block (was assembled) but I installed the gas tube as I built it.
Geissele Super 42 w/ H1 buffer

Perhaps I can get some feedback on how to properly troubleshoot it? Anything helps!

I have no first hand knowledge but I have heard several instances of people running the super42 and h1 kit and immediately getting short stroking.I like extra parts anyway so I would buy the most basic standard cheap spring and buffer and see what happens. You might also want to YouTube some school of the American rifle for quick easy bcg checks.

 

[DirectDrive]

Changing out to a standard carbine spring and carbine buffer would be an easy first step.
Also, be using full power ammo for your troubleshoot.
M855 or similar.

 

[Pnwbrrt]

Please describe your ejection pattern via the chart provided above. Check your BCG for leaks, test the seals (you should be able to set it on the bolt head without it unlocking), check the gas key, ensure there are no leaks from the gas block, ensure the buffer tube and spring are actually correct.

You can have similar issues on a system that is over gassed so the ejection pattern is an important detail.

It would be more likely that you're having gas leaks from your BCG than a misalignment of your pinned gas block so start from the butt and work your way up to the tip.

A great YouTube channel that does diagnostics, physicals and autopsies is "School of the American rifleman" if you want to fully geek out.

 

[CharlesAFerg]

Seems like I have some type of gas blow-by on one one side of the gas block, going forwards towards the muzzle. Thoughts?

 

[Pnwbrrt]

Seems like I have some type of gas blow-by on one one side of the gas block, going forwards towards the muzzle. Thoughts?

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It's bleed off. It reduces the amount of gas going into the carrier to what is necessary.

 

[msgriff]

It's bleed off. It reduces the amount of gas going into the carrier to what is necessary.

If you're implying that it's an adjustable gas block with bleed-off, you are incorrect.
OP, most gas blocks take some time to seal up, depending on fit. That one looks excessive, but will probably still seal.