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Discussion Starter · #1 ·
I built a 5.56 AR15 from an 80% lower and a CMMG upper and CMMG lower build kit. About once a mag the bolt locks back with rounds still in the mag. It's not jammed. All I have to do it hit the bolt release and it chambers a round and shoots again just fine. It has happened with three types of 5.56 ammo and both steel and poly mags. What might I be missing?
 

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I wish I could help, but all I can do is say to ask the experts. Here's a link to an AR forum that should be able to provide an answer for you. Hope it works out. AR-15 - AR15.COM
 
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Discussion Starter · #3 ·
I built a 5.56 AR15 from an 80% lower and a CMMG upper and CMMG lower build kit. About once a mag the bolt locks back with rounds still in the mag. It's not jammed. All I have to do it hit the bolt release and it chambers a round and shoots again just fine. It has happened with three types of 5.56 ammo and both steel and poly mags. What might I be missing?
Thanks for the link. I will wait for some people to chime in here first. I have not had good luck with other forums. To many trolls, jerks and know it alls. If I don't have any luck, I will post over there.
 

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It may be your bolt catch spring is too tight. How many rounds have you fired through it? How many rounds do you load in the magazine?

One thing I have noticed with many new guns is shooting will losen things up.
 

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Discussion Starter · #5 ·
There is about 150 rounds through it. I have always loaded 30 rounds in the mags. It usually happens between shot 10 and 20.
 

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Inspect the parts for damage (particularly the the pin itself and catch where it meets the pin. Manipulate the lever by hand and see if it catches/feels gritty. If all looks okay and the problem persists, replace the spring. If that doesn't fix it, I'm out of ideas.


 

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Discussion Starter · #7 ·
Inspect the parts for damage (particularly the the pin itself and catch where it meets the pin. Manipulate the lever by hand and see if it catches/feels gritty. If all looks okay and the problem persists, replace the spring. If that doesn't fix it, I'm out of ideas.



Thanks! Seems like a good place to start.
 
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Thanks for the link. I will wait for some people to chime in here first. I have not had good luck with other forums. To many trolls, jerks and know it alls. If I don't have any luck, I will post over there.
I certainly hear you on that. Some sit around waiting to snipe at the first person they can. TA is much more friendly. After giving your problem some thought, the only thing I can think of that would cause the bolt to stick on a partial mag is that the mag is seating to far in. However what would cause that I don't know.
 

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Did you get the right springs in the right place? I know the CMMG kit is color coded, but someone could have mixed them up somewhere along the line.
 
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Built out a stripped lower with a CMMG parts kit. I've had no problems. I doubt it is the parts kit.

On your issue, does the bolt lock completely open or does the bolt slam closed when you pull the magazine free. If the bolt slams closed, it isn't locked completely open by the magazine. This is a condition known as "bolt-over-base". This is caused by several things: insufficient gas to the gas key, buffer spring to heavy, buffer too heavy, underpowered ammo, to name a few. I would take it to a gunsmith and have it checked.

Another thought: are you running the Bolt Carrier Group with plenty of lube. Try cleaning it and relubing, then run a string of magazines through it. If it stops, run through the test above and see what happens.
 

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Discussion Starter · #11 ·
Built out a stripped lower with a CMMG parts kit. I've had no problems. I doubt it is the parts kit.

On your issue, does the bolt lock completely open or does the bolt slam closed when you pull the magazine free. If the bolt slams closed, it isn't locked completely open by the magazine. This is a condition known as "bolt-over-base". This is caused by several things: insufficient gas to the gas key, buffer spring to heavy, buffer too heavy, underpowered ammo, to name a few. I would take it to a gunsmith and have it checked.

Another thought: are you running the Bolt Carrier Group with plenty of lube. Try cleaning it and relubing, then run a string of magazines through it. If it stops, run through the test above and see what happens.

VERY interesting! I have no idea if the bolt would slam close if I dropped the mag. But next time it happens I'll be sure to check. Interesting guns AR15s. This is my first.
 

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Built out a stripped lower with a CMMG parts kit. I've had no problems. I doubt it is the parts kit.

On your issue, does the bolt lock completely open or does the bolt slam closed when you pull the magazine free. If the bolt slams closed, it isn't locked completely open by the magazine. This is a condition known as "bolt-over-base". This is caused by several things: insufficient gas to the gas key, buffer spring to heavy, buffer too heavy, underpowered ammo, to name a few. I would take it to a gunsmith and have it checked.

Another thought: are you running the Bolt Carrier Group with plenty of lube. Try cleaning it and relubing, then run a string of magazines through it. If it stops, run through the test above and see what happens.
With bolt over base, the bolt doesn't cycle back far enough and gets hung up on the top of the next round. He said he can hit the release and chamber the next round as normal, so I think it's an issue with the hold open engaging before the follower contacts it vs a BOB malfunction. The thing I can't figure out is why this happens only once per mag.
 

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I've built three ARs and one AK, but I'm following along in the hopes of learning more about them. Never too old to learn. I gave one of mine a close look yesterday in the hopes of figuring out what yours is doing, but for the life of me I'm puzzled.
 
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With bolt over base, the bolt doesn't cycle back far enough and gets hung up on the top of the next round. He said he can hit the release and chamber the next round as normal, so I think it's an issue with the hold open engaging before the follower contacts it vs a BOB malfunction. The thing I can't figure out is why this happens only once per mag.
This why I asked him to strip the magazine after the stoppage. If the bolt remains open, something else is wrong and it isn't a BOB stoppage.

I still believe a gunsmith who is a qualified AR-15 armorer will quickly find the problem and repair it. If it isn't BOB, then a weak or improperly installed bolt release spring may be to blame. The catch being actuated between 10 and 20 rounds may just be a magazine spring tension thing. Even so, if BOB is not the culprit, then the Bolt Release mechanism is the primary villain. :)
 

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I would ask about how freely the bolt release moves on the lower. When building up my lower, I did have to open up the slot for the bolt release a bit as it would bind up a bit. The binding bolt release will emulate the problems you're experiencing.
 

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The magazines could be a problem also. My AR pistol build was magazine sensitive.
 

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Nothing more from the OP? Has the issue been resolved?
 
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80% poly lowers CAN have minor casting tolerance issues. I pre-fit the spring and catch before the roll pin goes in. Everything needs to move freely - I opened the spring hole (probably .001!) on my last poly build to get it moving totally free.
 
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I just don't trust poly lowers (personal preference - we didn't have them in the Army, so why change a good thing;)) That being said, my AR did something similar, but not once every mag. More like once every third mag or so but after a few hundred rounds, it quit doing it :) - Definitely something to be said for the break-in period.
 
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