View Full Version : Rifle Gas Ejection/Lock Back/Gas Issue
swampfox44
04-16-19, 16:59
I have a Colt 6920 carbine I just swapped out the 16" factory barrel for a Larue Tactical Stealth 20" rifle length gas barrel. I installed a new SLR Sentry 7.750" adjustable gas block and new rifle length gas tube. I've gone through all 15 settings on the GB and finally got ejection at #13 but, no lock back. Stepped up to #14 and #15. Better ejection, no lock back. I'm wondering if I need to enlarge gas port or try standard, non adjustable block. My test firing was done with 55gr Pmc Bronze to get me in setting ballpark but, I don't seem to be getting enough gas to fully cycle the bolt. Any suggestions are greatly appreciated. Thanks
My first thought was...why did you do that? I live in Ca. And wish I could get an intact 6920.
Are you sure the GB is installed correctly? Are you sure you're adjusting GB corrctly?
IMO 20" midlength is a non-starter. I wouldn't be surprised if the gas port is too small.
swampfox44
04-16-19, 18:32
My first thought was...why did you do that? I live in Ca. And wish I could get an intact 6920.
Are you sure the GB is installed correctly? Are you sure you're adjusting GB corrctly?
Barrel is dimpled for alignment of gas block, when adjusted all the way open, setting 15, its ejecting but, still not quite enough gas. Thinking gas port size may be too small.
Looking at their website, the 20" is rifle length gas........
swampfox44
04-16-19, 18:36
IMO 20" midlength is a non-starter. I wouldn't be surprised if the gas port is too small.
I'm thinking gas port needs enlarging as well. If I'm at full open on gas block and still not enough gas then, step size up and try again.
Before you start drilling on your new barrel, measure the gas port...maybe a local gunsmith can help you there. I dont know off the top of my head what it should be, but others here do. If you have a fixed quality GB, try it.
swampfox44
04-16-19, 19:03
Looking at their website, the 20" is rifle length gas........
You are correct, posting mistake, it is rifle length. Thanks for the correction.
If that is the case, I am almost willing to bet that the gas block is not installed correctly.
You are correct, posting mistake, it is rifle length. Thanks for the correction.
A 20" rifle with a mill spec port will just run.
Assuming the port is good, it's likely the adjustable gas block is leaky.
Try a quality non adjustable gas block.
Something like this.
http://www.blackrivertactical.com/concrete5/store/#!/BRT-MicroPin-Gas-Block-750/p/17534540/category=6464008
swampfox44
04-16-19, 20:40
Port measured .087", drilled it out to .0938". Still same result, failure to lock open at full gas. I've got a new standard, non adjustable GB on the way.
So before doing anything else you went and drilled out the port?
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Port measured .087", drilled it out to .0938". Still same result, failure to lock open at full gas. I've got a new standard, non adjustable GB on the way.
My man...even though the port was under spec, we dont remove metal as a first resort. It might have worked with a fixed gas block. Always start with the less intrusive possible fix first.
While we're waiting for that new gb... does the bcg move freely in the upper, without any binding all the way from fully locked to fully retracted?
So before doing anything else you went and drilled out the port?
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Had to look that up....pretty funny.
The correct port size for a 20" rifle is well known, it was fine to fix it.
PMC Bronze not running in rifle length gas systems was a well known issue.
PMC Bronze not running in rifle length gas systems was a well known issue.
Good info from a while back:
https://www.m4carbine.net/showthread.php?7757-interesting-PMC-ammo-problem
Dare I ask what mag was being used ? And did you try multiple mags , pmags , lancer , 30 rounders?
PB
Had to look that up....pretty funny.
Ditto, pretty good.
swampfox44
04-17-19, 17:34
Yes, bcg smooth from retracted to locked and vice versa. Pulled block and gas tube, checked gas block port alignment, all good, measured port distance on barrel and block, both dead on, checked for fully open adjustment on gas block, it's wide open, unobscured, installed gas tube, checked for proper alignment to GB port, dead on unobscured. Checked gb boss on barrel for burrs or high spots, all flush. Installed gb back on barrel and tried 4 different known locking back mags with 3 different .223 loads including some hot Hornady Superformance and also full power 5.56. Ejects great, still no lock back. Checked around gb boss, didn't see any carbon blowby. So, waiting on standard gb to come in and try it.
swampfox44
04-17-19, 17:43
Dare I ask what mag was being used ? And did you try multiple mags , pmags , lancer , 30 rounders?
PB
30rd Pmag, 20rd Pmag, 10rd Colt, 30rd Colt, 30rd D&H, 30rd Bravo Company and a few misc USGI 30 rounders.
Honest question. Why didn't you contact LaRue knowing that the port wasn't correct and allow them to deal with it?
I have a Colt 6920 carbine I just swapped out the 16" factory barrel for a Larue Tactical Stealth 20" rifle length gas barrel. I installed a new SLR Sentry 7.750" adjustable gas block and new rifle length gas tube. I've gone through all 15 settings on the GB and finally got ejection at #13 but, no lock back. Stepped up to #14 and #15. Better ejection, no lock back. I'm wondering if I need to enlarge gas port or try standard, non adjustable block. My test firing was done with 55gr Pmc Bronze to get me in setting ballpark but, I don't seem to be getting enough gas to fully cycle the bolt. Any suggestions are greatly appreciated. Thanks
swampfox44
04-17-19, 18:38
It was a used barrel I picked up from a friend in a trade. Already had 1000 +/- rounds through it.
It was a used barrel I picked up from a friend in a trade. Already had 1000 +/- rounds through it.
So it was working fine for him?
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swampfox44
04-17-19, 18:50
So it was working fine for him?
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He didn't shoot it, stripped it and made an sbr.
swampfox44
04-17-19, 18:53
what weight buffer?
Factory Colt carbine buffer and spring. My 6920 was originally 16" carbine. Just swapped barrels and added adjustable gas block. Thought adjustable gb may have been needed to fine tune hot loads. I've got a new, non adjustable gb on order.
SF44...there's a good chance that it's the wonky PMC Bronze you're using like Todd K said. Put it together and try a different ammo with more horse power. I remember chronoing Bronze out of a 20" at only 2850 fps...really weak. Its evidently got a suspect pressure curve for a 20 also.
swampfox44
04-17-19, 19:34
SF44...there's a good chance that it's the wonky PMC Bronze you're using like Todd K said. Put it together and try a different ammo with more horse power. I remember chronoing Bronze out of a 20" at only 2850 fps...really weak. Its evidently got a suspect pressure curve for a 20 also.
I ran 4 different loads today, pmc bronze 55gr 223, federal 55gr 223, hornady 223 superformance 53gr, and american eagle 5.56 55gr. Still same, good ejection, no lock back. Got a non adjustable gb ordered. Going to try it when comes in and go from there.
This is a tough one. Did you check for any bcg binding from full lock up to full retraction? Also, does the bcg lock back on an empty mag when you operate the action by hand? It probably all worked fine before the barrel change out.
swampfox44
04-17-19, 20:23
This is a tough one. Did you check for any bcg binding from full lock up to full retraction? Also, does the bcg lock back on an empty mag when you operate the action by hand? It probably all worked fine before the barrel change out.
Yes, bcg operates smooth, yes, bcg locks back on empty mag with charging handle, yes, everything ran fine before barrel swap and addition of the adjustable GB. I'm also considering the 6920 may not be broken in yet. I've only put about 40 rounds through it since new. Considering that 16" carbines are borderline overgassed, it probably had more than enough gas to reliably cycle. Thinking maybe with the substantial pressure difference between carbine gas and rifle gas, rifle may need to get worn in. I'm going to install the new gb when it comes in and test. If still no lock back, I'm going to liberally lube the bcg and start running ammo through it.
The reason I ask about those things is I've seen guns that have had the gas tube bent to the side when built or rebuilt binding the bcg. Hand locking it helps eliminates any probs with the lower.
When I was building my 3gun rifle with an 18" rifle gas barrel and new bolt I encountered short stroking. It turned out to ;e the new tight gas rings. I hand cycled the action a bunch and that fixed it.
swampfox44
04-17-19, 20:56
The reason I ask about those things is I've seen guns that have had the gas tube bent to the side when built or rebuilt binding the bcg. Hand locking it helps eliminates any probs with the lower.
When I was building my 3gun rifle with an 18" rifle gas barrel and new bolt I encountered short stroking. It turned out to ;e the new tight gas rings. I hand cycled the action a bunch and that fixed it.
Ha! I was actually thinking of lubing the bcg and hand cycling it alot as well. I had a Wilson Combat 1911 I hand cycled and dry fired alot and it smoothed out the trigger and action.
Dr. Bullseye
04-17-19, 21:18
Adding an optic or sling mount is doing cosmetic surgery on the rifle. But the OP did brain surgery on his. This is no different from any other machine. Engineers spend millions designing and testing and evaluating until they reach as workable balance. Doing brain surgery on a car or a boat or even a house is full of unintended consequences.
Still same, good ejection, no lock back.
What do you call good ejection?
My first impulse, based on 'good' ejection and no lock back would be that you are outrunning the spring on the mag due to running a carbine buffer on a rifle system. But if that was the case, you'd be having fail to feeds or bolt overrides, which you don't report.
By my figures you increased the area of the gas port about 14%, I'm not smart enough math wise to figure the increased pressure, but I'd reckon it should be enough to get the BCG back far enough to lock on one of the various mags you've reported using.
Shotgun the upper open and manually, with your hand, move the bolt back and forth to make sure that the gas tube fits into the carrier key smoothly - something you should have done upon assembly;
I believe you said you've checked the gas block and gas tube at the gas block for signs of leakage, if not check those;
Check the bolt carrier in the area of the gas key to see if there is any sign of leakage - carbon venting from between the carrier key and the bolt carrier;
Check the gas rings, it is possible that Colt messed up, while your at it check to see if the bolt retracts into the bolt carrier under the weight of the bolt carrier. (ETA: it shouldn't)
If all that checks out wait for the new gas block.
A few observations.
1. I wouldn't be using PMC Bronze to perform a check.
2. Get a real gas block.
3. A Carbine buffer and spring isn't the ideal set up for this. The closest in weight to the rifle buffer is the H3. If your gas port is truly at least .093 it should run with an H2 or H3.
4. I am also curious to know if this barrel is the standard one or has the M4 feedramps.
swampfox44
04-18-19, 19:27
Finally found the problem!!! Machined aluminum shaving wedged in gas key and tiny aluminum flecks in bolt area of bcg. Took gas block apart and still had trace glitter from machining. Cleaned gb, blew out with compressed air, reassembled, cleaned and lubed bcg. Locks back on setting 4 for hot Hornady loads and XM193 and setting 7-9 for american eagle, pmc and remington .223. This thing was driving me crazy. Thanks to everyone for all input and suggestions.
Wow, any idea where it came from? I've heard of primers bits winding up in there. Didnt see that coming.
Reread your post...came from th adjustable gb?
swampfox44
04-18-19, 20:10
Yes, only machined aluminum in gas system is the adjustable gb. It had traces of aluminum glitter still in it. Guess bad quality control. Already got 2 emails into SLR Rifleworks and a phone call. No response as of yet. Looks like GB housing wasn't thoroughly flushed/cleaned after machining before assembly. But, main thing is its running.
swampfox44
04-18-19, 20:14
Update to that, the GB is 4140, not aluminum, I still have the contaminants, will have to put magnet on them tomorrow. Coloring sure looked aluminum.
Already got 2 emails into SLR Rifleworks and a phone call. No response as of yet.
SLR will take care of you. They've got good customer service.
swampfox44
04-18-19, 20:32
It was a learning experience, not thrilled with findings but, not bitter, just want to make them aware of potential issue with their product. One I got could have been only one, who knows. I'm just glad I found issue.
Are you sure that the debris wasn't from the gas tube?
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