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Posted: 8/22/2011 3:09:31 PM EDT
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I built an AR a couple yrs ago, it's just been sitting around because it never has cycled correctly. Decided to try to fix it, here's the deal:
The gun shoots fine, ejects well, but the bolt gets stuck in the half-open position. It has already pulled another shell out of the mag, but the bolt doesn't close forward. I've swapped uppers and lowers, the problem follows the lower. When firing with a mag with only 1 shell, the bolt locks open like it should. I'm using mags that work fine in my other AR. I've replaced the butt stock with another that I had, same problem. It looks like the problem is in the the lower, maybe the trigger/hammer parts. Am I missing something? Should I take apart the lower and re-assemble? Should I compare all the parts to another lwr parts kit? (I have another one because I'm planning to build another one). Thanks for the help, as you can see, I'm not an expert. |
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[Push in the mag release with a tool of some sort, then tighten the piece on the opposite side as far as you can (it turns like a nut on a bolt, clockwise to tighten).]
Thanks for your help. I will try that. So even if the mag release is working correctly, it could cause the bolt not to cycle forward completely? Are you thinking that the mag isn't being held tight enough? I'm just trying to understand. Thanks. |
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Quoted:
It could be that your recoil spring is weak. Try swapping recoil springs and buffers. This. Is the bolt fully stripping the next round out of the mag, or is the bolt getting jammed on the next round? If your buffer spring is weak/worn, the bolt can actually cycle faster than the magazine can feed. Look at the casing from when the bolt hangs up. Do you see marks where the BCG digs in? Buffer springs are cheap. Replace and let us know. |
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Replacing the trigger may resolve the problem, so long as the FCG receiver holes are in the correct location, since what happen with some triggers is the distance from the center of trigger pin to front of sear is too short, allowing the hammer to be too raised at retention, and allows the L cut hammer top of contract pad edge to catch the FP collar on a Un-shrouded carrier to lock the bolt up half way home.
If you just want to tweak the parts if the above is the problem, the start with the FP to make sure that the stop collar on it is flush or slightly below the ramp of the bolt when it it pressed all the way forward with the bolt in the rearward position (spin file the collar down if needed), then round the top of the hammer contract pad edge to radius the edge (again, just round the edge, and do not try to lower the top of the contact pad on the hammer. |
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I swapped the stock with a proven stock, same thing. It still locks back when shooting with only 1 round in the mag.
Also, with a round chambered - and the mag removed - it still happens. Seems like the problem has to be in the trigger group. Will post back when I figure something out. I'm building another lower and will disassemble this one at the same time. If anyone has any other ideas, please advise. Thanks |
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Quoted:
I swapped the stock with a proven stock, same thing. It still locks back when shooting with only 1 round in the mag. Also, with a round chambered - and the mag removed - it still happens. Seems like the problem has to be in the trigger group. Will post back when I figure something out. I'm building another lower and will disassemble this one at the same time. If anyone has any other ideas, please advise. Thanks When you say you swapped the stock, what exactly did you swap? Becasue the butt stock is not going to change the way the weapon functions internally. Do you perhaps mean you swapped the buffer spring, buffer, or buffer tube? The previous poster was refering to the buffer spring, which is within the tube that the stock mounts on. You remove it by depressing the detent behind your fire control group at the bottom of the tube, and pulling the buffer and spring out. Also, I usually put a light coat of a dry grease (lithium based or synthetic) on the spring to help cycling and keep it quiet during operation. |
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Do you have a ramped bolt carrier, meaning that on the bottom it is cut away to expose the firing pin? Also does the hammer have a shelf cut across the top front (where the top of the hammer meets the front of the hammer)?
CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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Yes, the hammer has a shelf cut into it. Just looking at it compared to a functioning lower, it all looks the same. And the problem is isolated to the lower, this lower does the same thing with another upper, while the upper functions fine with another lower (hope I said that right).
I've started building another lower, and am going to take apart the trigger/hammer group at the same time and see if I can find the problem. Thanks |
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Again, when you get the new hammer for the other kit, compare it to the questionable lower hammer.
Your looking at the distance of the trigger from the pin location, to the end of the sear. When such is short, then the hammer is retained higher in the receiver on the trigger, and can catch the FP top collar. |
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I am thinking of two possible causes, the disconnector is worn or out of spec, or possibly the disconnector doesn't have any or enough spring tension. If you are using a single stage trigger, then check to see that there is a spring under the disconnector. If the disconnector is not engaging the hammer, then the hammer is riding the bolt carrier forward, of which if you have a ramped (exposed) firing pin on the bottom of the bolt carrier then the hammer will lock into the bottom of the firing pin, which will cause the action to lock forward about 1/2 way.
As a possible way of diagnosing the rifle, try this function check with the entire rifle assembled and no ammunition or magazine in the rifle (unloaded): 1. Cock the hammer by cycling the action to the rear. 2. With the safety in the "Safe" position, try pressing the trigger. The hammer should not release. 3. Put the safety in the "Semi/Fire" position, and press the trigger. The hammer should release from the hammer/trigger sear engagement, and go forward striking the firing pin. 4. Release the trigger, then manually cycle the action. The hammer should return to the hammer/trigger sear engagement, so try pressing the trigger again and it should go forward again and strike the firing pin. 5. Press the trigger to the rear and keep it pressed to the rear. With the trigger pressed to the rear, manually cycle the action rearward and release it (letting the action travel forward under its own spring pressure). Now release the trigger, you should feel and hear the hammer release from the hammer/disconnector sear engagement, and then the hammer should go back to the hammer/trigger sear engagement. 6. Press the trigger again and it should release from the hammer/trigger sear engagement. CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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Mater, forgot to ask up front, then you get the jam, is the bolt behind the back of the rim, or sitting on top of the middle of the case (read when you pull the round, is there bend/dent on the middle of the case?
I'm thinking that is the trigger from pin location to end of the sear that is too short, causing the hammer to be retained to high when on the trigger sear, allowing the L cut top edge in the hammer to catch the firing pin when the B/C goes forward. While Sully is thinking that the disco is not catching/retaining the hammer (both will cause the same problem), but if you are having a short stroking problem, then could just be a bad feed with the bolt not getting behind the round in the mag in the first place. If you are getting middle dented rounds on the jams (bolt face against the middle of the case, and not behind the rim on the jam), then you will need to go back to square one with the lower build. Could be that you have the wrong buffer or spring in play, or even Hammer wedging against the disco/back of trigger that is causing the short stroke. |
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THANKS SULLY
5. Press the trigger to the rear and keep it pressed to the rear. With the trigger pressed to the rear, manually cycle the action rearward and release it (letting the action travel forward under its own spring pressure). Now release the trigger, you should feel and hear the hammer release from the hammer/disconnector sear engagement, and then the hammer should go back to the hammer/trigger sear engagement.
Hey Sully - this is where the problem is. When I keep the trigger squeezed and cycle the action rearward and release, the action only travels halfway - then it hangs up. Thanks alot. I haven't been able to replicate the problem in the basement until now. So should I go ahead and take the trigger and hammer assemblys out and compare all the measurements? Mater |
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Sorry - I didn't read everything. Is this likely the problem?:
If the disconnector is not engaging the hammer, then the hammer is riding the bolt carrier forward, of which if you have a ramped (exposed) firing pin on the bottom of the bolt carrier then the hammer will lock into the bottom of the firing pin, which will cause the action to lock forward about 1/2 way. Mater |
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Remove the upper from the lower, aim the front of the receiver upwards, cock the hammer back past the disco with the trigger pulled back, then release the hammer. Did the disco catch/retain the hammer, hence when you cocked the hammer past the sear with the trigger pulled back, was there tension on the disco as the pressed the hammer past it?
Now with your hand in front of the hammer, do the trigger release now to see if the disco releases the hammer, the front trigger sear catches the hammer? Also check to make sure that the disco is moving cleanly in the trigger/not binding up in the slot or on the pin. Lastly here, I can see that the pins match the receiver, but make sure that the FSG parts match the pins as well (read do not have a .174 colt FCG parts riding on .154 mil spec pins). Now with the trigger rest, cock the hammer back past the disco again, and tell me the free gap between the disco and back hammer sear. Now on the FP in the carrier, look at the stop collar on the FP and make sure that the collar is not proud of the carrier ramp when the FP is pushed all the way forward with the bolt in the open position. 
Lastly, with the bolt removed from the carrier, dry fit the carrier into the upper receiver all the way forward to make sure that the gas tube is aligned to the carrier key. |
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Guys,
I really appreciate all the help. In taking apart the problem lower last night, I discovered that I had neglected to install the disconnector spring. I'm sure you all were working under the assumption that the gun was complete. Sorry for the time you all spent to help, but I appreciate it. I'll shoot the gun tonight, but I'm sure that this is the problem. Thank you, Mater |
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Quoted:
Guys, I really appreciate all the help. In taking apart the problem lower last night, I discovered that I had neglected to install the disconnector spring. I'm sure you all were working under the assumption that the gun was complete. Sorry for the time you all spent to help, but I appreciate it. I'll shoot the gun tonight, but I'm sure that this is the problem. Thank you, Mater Was the disconnector in backwards as well? 
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Hey Guys,
Took the hammer & trigger out, found a disconnector spring, replaced it all. Shot the gun, it works fine. Thanks for your help, taking your alls advise helped me look at the operation of the gun and see that the disconnector had some play. Thanks again, Mater |
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Glad you got it running correctly.
CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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