AR Sponsor
Posted: 10/5/2012 7:08:43 PM EDT
|
I run our rifle program at my agency. I'm in charge of maintenence on all 400-odd rifles, 2/3 owned by officers, 1/3 by the department. I'm not a hack, I've been working with ARs for long before I was a cop, and I've been to a number of AR schools, and I can do a lot of things that go beyond armoring and into gunsmithing. Anyways.... Officer H's personally owned rifle is a Colt 6920LE. It is 2-3 years old and has no more than 2k rounds through it. No modifications other than magpul stock and pistol grip. Officer H is former .mil, and a pretty decent shooter. Doesn't shoot a lot by our standards, but for a police officer, she's out on the range every couple weeks shooting 50 rounds, so pretty good for a cop. I don't belive she would have done anything stupid to it (other than one thing mentioned below). Recently, the rifle began having feeding / cycling issues. The bolt would fail to chamber the top round, either closing on an empty chamber or stopping halfway closed (photo). This began occuring several times each magazine.  Most of the ammo being shot was department, steel cased Hornady 55gn practice. I know its steel, but its loaded hot enough where that shouldn't be an issue. We have actually had good luck with this ammo. We did have the problems with other, brass cased ammo as well. I do not think it is an ammo problem. It did not seem to be mag related, but to be safe, we swapped mags and the problem continued. The first person she talked to had a "friend" who was an "armorer" who told her to clean the gas tube with a pipe cleaner  . Once we discussed that, I took a look at her gun.It seemed to most likely be a gas issue. Gas rings seemed fine. Gas tube "nub" where it enters the gas key showed wear on the topside (only visible when removed), to the point where the nub was smooth with the rest of the tube. This seemed, at the time, to be the culprit. Replaced gas tube, and the issue remained. Also replaced gas rings, issued remained. I inspected all applicable parts for signs of rubbing (carrier, bolt, buffer retainer, action spring, buffer, receiver extension, etc). Found none. I shot her upper on another lower - NO GO. I shot her lower on another upper - GO. She told me she has never used foaming bore cleaners or anything like that on her rifle which would suggest the gas port / gas block could have become clogged. I then began to swap parts in an attempt to isolate the problem: OFC H = Officer H's gun DEPT = parts from another dept gun ME = my personal 6920 work gun which I know 100% works and is well cared for. BOLT.....CARRIER.....UPPER.....LOWER..... RESULT OFC H..... OFC H..... OFC H....... OFC H..... NO GO OFC H..... OFC H...... Dept......... OFC H.... NO GO OFC H...... OFC H.... OFC H.......... DEPT.... NO GO OFC H...... OFC H..... DEPT.......... DEPT..... NO GO DEPT........ OFC H..... DEPT.......... DEPT..... NO GO OFC H....... DEPT...... DEPT.......... DEPT...... NO GO DEPT......... DEPT...... DEPT........ OFC H........ GO OFC H........ OFC H..... OFC H....... DEPT...... NO GO OFC H......... ME......... OFC H........ OFC H..... NO GO ME............ OFC H....... OFC H....... OFC H...... NO GO ME............. ME.......... OFC H........ OFC H......... GO OFC H....... OFC H....... ME............. ME........... NO GO OFC H........ ME........... ME............ ME............. GO** cycled & chambered, but did not lock back on last round in mag ME............. OFC H..... ME............ ME............. GO** cycled & chambered, but did not lock back on last round in mag The cool thing with being a department armorer: I get paid to "trouble shoot" - (literally) and don't pay for ammo  Anyways, as you can see, anytime Officer H's bolt, OR Officer H's carrier were involved, there were problems. When both Officer H's bolt and carrier were involved, the problems seemed to be happening more frequently. The problem will not neatly narrow itself down to a bolt OR carrier issue. It is both. I closely inspected the bolt and carrier under a magnifying glass. No cracks or anything visisble at the cam pin hole or at the lugs. Gas key is secure and staked of course. -Can't be an out of spect chamber because this is happening with her bolt/carrier on other uppers. -When we shoot brass cases, I have found no evidence of overpressure. She said she didn't have any problems with the rifle until recently when it began happening suddenly. Now, the simple answer is of course to treat the symptom and replace the bolt and carrier. That is what we will do, but, I thought I would throw this out there for people, like myself, who like gun puzzles. I of course like to get a definitely answer. On this one, I may not have one. Thoughts? |
| I would inspect the gas key again. It seems like one of the screws may have broken off internally and the bolt is not getting a full dose of gas. The staking may be holding the screw in place. The clue is that your bolt and BCG work with her upper. Remove the gas key screws, clean and replace gas key using new screws, re-torque and stake the screws. |
|
Quoted:
I would inspect the gas key again. It seems like one of the screws may have broken off internally and the bolt is not getting a full dose of gas. The staking may be holding the screw in place. The clue is that your bolt and BCG work with her upper. Remove the gas key screws, clean and replace gas key using new screws, re-torque and stake the screws. +1 |
|
If it happens with office H's carrier and bolt - for ~ $150 just get a new bcg and be done. I would think you have the tools and can do a good carrier key stake but it may be just as easy to get a new factory complete BCG. Though this is a Colt - Colt might replace it for free under warranty.
|
| On the outward facing portion of bolt that is visible thru the ejection port when the bolt is closed there is a small gas port in the saw toothed area that the fwd assist engages. This port is slightly forward of the hole that the firing pin retaining cotter pin sits in. Check to make sure that this is not carbon fouled and partially/completely blocked. |
|
Thanks for the ideas guys. I hadn't thought of the possibility of one of the screws breaking, but not being able to tell because of the staking. We ordered another BCG for now to get it up and running. I'm on vacation for a few, but I will take a look at it again when I get back. Thanks for the ideas.
|
AR Sponsor