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AR 15 Build Gas Problem

richidaho

Sergeant
Full Member
Minuteman
Feb 6, 2014
215
104
Blackfoot, ID
I have built 12-15 AR's in the past with very few problems. I had been buying most of my parts from surplus ammo and arms. I switch to Palmetto arms for these last 3 builds 2, 6mm ARC's. The ARC's worked fine. I used a 20" Nato 5.56 that came from midway AR-Stoner 1-7. I put on a rifle-length gas block and tube. I used the Palmetto PSA AR15 mo2 lower build kit (51655120182). This has a collapsable stock and carbine buffer system. Took the gun to the rage. Put one round in the clip shot the gun would not lock back. Checked the gas system, checked the buffer, spring, and bolt. Retryed bolt would not lock back. Took the gun home and ordered an Oden works adjustable gas block and installed it. Ended up adjusting the gas block all the way open; the same problem. I have tried other buffers, springs, and bolts same problem.
The gun will cycle but will not lock the bolt back and looks to be under gassing. Brass is kicking out at about 3-4 o-clock. It should be 4-5 o-clock by what I have seen and read. What should I try next? Gas tube? Looking for some tips.

Thanks,
Rich L
 
Does the bolt lock back if manually retracted with an empty mag inserted?
Does the gas tube protrude the correct length into the upper receiver? Sorry I don’t have a dimension. See pic.
Gas port on barrel in the 12 o’clock position and the proper position on the barrel?
14E28F39-CB7C-4010-9C46-C84B65F8D195.png
AC153419-BC8F-46C2-84BF-D7809DED29E0.png
 
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It does lock back with an empty mag. I have attached photos of the tube and block.
 

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Looks right. I would remove gas block from barrel to check the hole location.
And you did try a bcg that is known to work as a test also?
 
3-4 o'clock ejection is good. 4-5 o'clock is under gassed.

When you say it does lock back with an empty mag, are you pressing on the bottom of the bolt catch then pulling on the charging handle to lock it back to the rear or are you just inserting an empty mag into the lower then pulling charging handle to the rear or are you just inserting it into the lower (without upper) like pic below.

There should be ample material for the magazine follower to push on the bolt catch. Even if it looks like there is enough material attempt these steps
1st Test
- lock the bolt to the rear
- Insert empty magazine forcefully.

2nd Test.
- Lock the bolt to the rear with empty magazine in lower.
- Smack the bottom of the magazine upwards forcefully

Results for both test should be -
Successful - BCG stays locked to the rear with both test 1 and 2.
Failure - BCG goes forward on either test 1 or 2.

Why would it fail? Could be due to tolerance stack issues with lower and bolt catch, weak bolt catch spring or binding spring, worn magazine follower, weak magazine spring. Change 1 item at a time starting with a brand new magazine and repeat tests.

If tests are successful, revisit the gas system.
4KF-UewKAOPsGqvqrKrQFXl24ZMMJuqYpkTa0jy5RHLlHVM94mMwwKBMJzEj2TIYMZuYQO1SbtKNxlb9WEuYqd4uSpWCbRorfbXIuGrPxH3BWoMqjh0g_p5YJP5CvJ1PRs20hPtBlXMyMWG2jTwlnuj6OKQ8YS9UaMXZxF-_5n610z5Iv9kZmXto5rU1m4WS1Z5HFihFcjQnWZVaHwncvRmokXYaYcKpzH_n8uMlTlJvLetB9o_R2Yxe9AeB7DwzSVK6ZDUX5ReYJEOGnIMSyyKHfpAlJyQZANy_7kdqRhpV7XL2mpuSLP1nWwvmlrF5AJVEg_adUVYwFFdL-LndN2xEYKXpuHOqVoUCW3VZPgM_nTrglRb_y7VTE2f39cK2dBtoY1NZMCuC1IMBbZeuXo4HffbPEqKU4y7qgKJPuCtqrrcWl59Sy0CKPp6UDyh_xXMJ59vGnCSvMTs_xrYmkBSGWtTxreEN2lgNO0vzVvaGXYOf1Na9C2mt3YlmZWYMA_s-57HPyAwHXWp-kkqV8H4VU4WARfE-ctmIKIIK6Gm8DellrDo157efmaXvnbZOqMqyzD2ZT0HQHZzFUBlBQCqg_fJ-XtlaxJZ0vNbgmuYwNxIPXXSZDlqGizKgDBvL2F-5R4Jf15wjcW_zyZgzhCu4No_yk_td_j4ozp-o5uQ-8Xz91FQzDLdaZb-FVVqXvFoc2_-QfMv-uxuwuqu52ZI=w726-h1289-no
 
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Very rare, but broken gas key screw. Difficult to test without disassembly, which you would have to be prepared with new screws and re-staking. Not the first thing I would look for though.
 
Very rare, but broken gas key screw. Difficult to test without disassembly, which you would have to be prepared with new screws and re-staking. Not the first thing I would look for though.
He tried a different bolt carrier group which did not function either.
 
What ammo are you using and have you tried other?

If you have access to a known-good BCG, try that. If the rifle runs properly then you know the BCG is the issue and you need to disassemble, inspect, and perhaps take measurements.
 
So, checked the gas keys and tried it with a working BCG. Just noticed this. This bolt lock looks damaged. But it did pass Evintos's tests.
 

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So, checked the gas keys and tried it with a working BCG. Just noticed this. This bolt lock looks damaged. But it did pass Evintos's tests.
The leading edge of the catch looks peened, like it's getting hammered by the bolt or carrier when raised and the BCG cycling rearwards.
 
When you go test the upper with the new gas tube, if you still encounter failure to lock back, try the upper with a completely different lower. If it works with a completely different lower, even with a 3-4 o'clock ejection pattern even with swapped gas system parts, even with a passed manual lock and jolt test, I would start suspecting a crooked bolt catch recess, crooked mag catch or mag catch slot or crooked magazine well rather than a gas issue.

The bolt catch lug peening is rather concerning and it should probably be replaced. The bolt catch lug should not be contacting the bolt lugs or carrier. It should be well underneath and between the bolt lugs as the BCG moves backwards.

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If there's no contact between the bolt lugs and bolt catch lug, try this test - insert an empty mag into the lower and wiggle the magazine side to side then wiggle the follower, followed by wiggle the bolt catch to see if you can get the bolt catch to slip off the follower like so. If it does slip off, replace the bolt catch, repeat. If it does so with a brand new bolt catch, replace mag catch, repeat test. If it continues to happen, I would lean towards improperly machined lower.
1645224402027.png
 
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I skimmed over this and didn't see it mentioned, need to measure the gas port in the barrel. sounds like it may be too small. The bolt catch getting beat up is a new one on me. Never seen that before.
 
I’m unsure if the bolt catch was not damaged before installed. I did not look at it very closely. Never had a problem with one before.
Also, thanks for all the advice. It helps me know if my thought process are working right. And thanks again. Need to make me a flow chart.. lol
So my current plans is change bolt catch and magazines release. Parts ordered.
 
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I skimmed over this and didn't see it mentioned, need to measure the gas port in the barrel. sounds like it may be too small. The bolt catch getting beat up is a new one on me. Never seen that before.
This. Rifle length gas tube and unknown gas port size + carbine buffer system = possibly not really designed to go together.

The only rifle length gas/carbine buffer combo I have is my PRI Mk12 build...which was designed to work with carbine buffer system.

This is the hard part about mix/matching "mil spec" because "mils spec". Mixing rifle length stuff and carbine length stuff can cause issues.
 
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And you've taken it from 0 to 100% on the adjustable block with the lightest buffer?
Next time you have the gas block off try to measure the port.

What ammo are you using?
 
Not completely related, but...

My Aero M5 started having very similar problems.

Tried a different magazine - nope.
Tried a different buffer - nope.
Tried a different buffer spring - nope.
Changed out the gas block and installed an adjustable gas block - success.

But wait, there's more!

After a spell, the adjustable gas block was giving me problems. The dealer who built the rifle had a newer M5 on the rack, and showed me the adjustable gas block they used on that build. That gas block had an adjustable detent on the adjustment screw. Told him that's what I wanted and to order me one. Got the new part, changing out the old gas block, I noticed one of the small screws had completely backed out and went for a walk somewhere.

Everything seems good now, but damn, talk about playing whack-a-mole with parts.

But I learned a shit ton referencing threads here and working on the rifle. Win win.
 
I had an issue where my bolt wouldn’t lock back on my SBR. Opened the block all the way doing the 1 round at a time. The brass ejection and feel of the gun just screamed overgassed

Found out it was a faulty Magpul mag. One I just grabbed from my parts bin. Rifle ran everything fine before that and everything since. I adjusted it back to original setting after noticing the mag was the issue
 
Well, she is running now. I changed the bolt catch and mag catch. No problems now. I'm 95% it was the bolt catch (see photo above). I will have to look at my parts a bit better next time and man is it a bich to get that bolt catch cotter pinout.
 
Ugh..
This is an old wives tale that needs to die.
Right. Because carbine length, mid length and rifle length gas ports are all the same size, and the pressure in the barrel/at the port/dwell time are all the same therefore all the parts related to the gas systems can be interchanged.
 
Right. Because carbine length, mid length and rifle length gas ports are all the same size, and the pressure in the barrel/at the port/dwell time are all the same therefore all the parts related to the gas systems can be interchanged.
Sarcasm is funny!
 
Right. Because carbine length, mid length and rifle length gas ports are all the same size, and the pressure in the barrel/at the port/dwell time are all the same therefore all the parts related to the gas systems can be interchanged.
All correct. Good thing the buffer system isn't tied to any particular gas system.