9mm AR-Bolt sometimes stays open on loaded mag

[minutemen1776]

I have an issue with my CMMG 9mm AR. This carbine has functioned well until recently. Yesterday, I took it to the range and discovered that the bolt would frequently (but not always) lock back during firing even with a loaded magazine in place. When this first happened, I looked inside but saw nothing obviously wrong. With each occurrence, simply pressing the bolt release closed the bolt and the gun fired as it should. This happened with three of the four mags (all Metalform) I was using. The fourth mag was no different than the rest, and all four have functioned fine until now. So, what am I dealing with? Could a weak spring somewhere be allowing the bolt catch to creep upward and engage the bolt when it shouldn't? I'd appreciate any helpful input.

 

[dogmush]

Does the CMMG use a magwell block? Mine 9mm AR would stick open while the block was too high during the adjustment phase of the build.

 

[minutemen1776]

It's a dedicated 9mm. There's no magwell block.

 

[HJ857]

There's no way the bho could be engaging while there are rounds in the mag since the bho is part of the follower.

I would look first at the back of mags themselves to make sure that they aren't getting beaten out of shape and blocking the movement of the bolt or otherwise engaging the lower's bolt catch.

 

[dogmush]

hmmmm. You could check the bolt catch spring, but they are kind of a pain to get out without scratching stuff.

Check how the mags are locking ing and running with the upper detached, then check to see if anything is rubbing on the bottom of the bolt.

 

[eldon519]

Just out of curiosity, have you tried cleaning it really well? I doubt it's the problem, but I know 9mm ARs get pretty nasty in there.

 

[minutemen1776]

My reference to the bolt hold open should have been the bolt catch. You're right that the bho could not be the culprit; it was a good 10-15 rounds down inside the mag.

This guy describes basically the same problem, also with a CMMG gun:

http://www.reddit.com/r/ar15/comments/200cf3/problem_with_9mm_ar_bolt_hold_back/

I will check the backs of the mags when I get home. Still, I don't understand why this problem would come up all of a sudden. It was actually very clean when I took it out yesterday.

 

[wally]

Have you started shooting resting it on the magazine or holding it by the magazine? Sometimes its possible to over insert the magazine because the catch cut out is a good bit wider than the catch. Holding or resting on the magazine can make it "randomly" be over inserted.


I've had FOD interfere with the bolt catch spring so it wouldn't move down and thus prevented the bolt from locking back on the last shot. I guess it could interfere in reverse and be keeping the spring compressed fully down allowing recoil to randomly active the bolt catch.

 

[HJ857]

Be sure to update this thread when you find a solution, I run the same lower and am interested to know how it turns out.

 

[minutemen1776]

I separated the receiver halves last night and looked at everything. Again, nothing appeared out of the ordinary. There was no sign of the bolt rubbing. I inserted a few empty mags, and everything looked fine. Loaded with a single round, a few mags left very little space between the hold open tab and the bolt catch. However, with more rounds loaded, there seems to be no reason for the bolt catch to engage.

Examining the bolt catch a little closer, I thought it more and more likely that the trouble is a weak spring. There was a lot of slop with the bolt catch, and very little spring pressure holding it down in its normal position. I could easily see where the vibration of firing the rifle might make the bolt catch jump upward and engage the bolt. So, I taped up my receiver and carefully drifted out the bolt catch pin.

Upon removing the bolt catch (something I've done on another rifle once before), I expected that the plunger and spring would freely pop out. But that did not happen. Instead, the plunger appeared to be stuck in a downward position. Pressing on it with a punch, it would move only a little. After manipulating the plunger for a good while (using oil, a butane lighter, a mallet, pin punches, a dental pick, and eventually a small drill, all of which were suggested in various Internet postings as a means to remove a stubborn bolt catch plunger), the plunger eventually came free, allowing me to remove it and the spring beneath it.

I saw nothing about the plunger or spring to suggest why it became stuck. Since I damaged the top of the plunger while removing it (though miraculously not the receiver itself), I will simply get a new spring and plunger. I will hopefully get those by Monday. In the meantime, I will thoroughly clean the hole and look for any burrs or problems with the receiver. If everything checks out, I will reassemble with the new parts and pray this solves the problem. With any luck, I'll find some time next week to get back to the range to do a function check.

 

[eldon519]

Thanks for the update, I would have to imagine that was the problem. Were there any burrs in the hole?

 

[minutemen1776]

UPDATE: Problem solved

I received the new parts Tuesday and installed them Wednesday. When I examined the spring/plunger hole, it appeared to be free of any debris or burrs. I went ahead and blasted it with compressed air just to be sure. Looking more closely with a bright light, I noticed a slight protrusion of the metal near the lip of the hole. Frankly, though, I don't think this aberration was pronounced enough, or in the right place, to cause the plunger and spring to get stuck. Nonetheless, I tried to gently smooth it by honing it with a small steel Allen wrench (I didn't have anything resembling a file that was small enough for the tiny hole). I don't know how much that really accomplished, but that area did seem a bit more polished in the end. After that, I put a drop of oil on the plunger and spring and installed them. Both moved freely in the hole, as they should. Finally, I reinstalled the bolt catch and drifted its roll pin back into place. With the new parts, the bolt catch at rest was held in place with the correct amount of spring tension.

Friday, I went to the range in spite of the mid-90s temps. Using the same mags I'd used before, I ran about 75 rounds through the rifle. On the first mag, the last round failed to feed. This had nothing to do with the bolt catch, and I determined the issue was with the magazine. I tried to replicate the jam several times by loading just a few into that same mag and then firing until the bolt locked back. I never encountered another problem, so go figure. After that, I emptied all the other mags without a single problem. The bolt locked back on every empty magazine, and it never locked back with rounds loaded in the mags. So, it appears replacing the plunger and spring did the trick.

 

[eldon519]

Glad you got it fixed and thanks for coming back with the update!

 

[HJ857]

Yes, thank you for the update!