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12/6/2019 7:27:02 PM
Posted: 3/9/2011 4:08:53 PM EST
[Last Edit: 3/28/2011 2:00:03 PM EST by Jaberie]
My CMMG M-4's bolt will not completely close and hangs up when going into battery at end of the carrier stroke. At the carrier's forward position, the bolt is jamming and is very difficult to retract (yes, that means a buttstock-ground bang with a firm grip on the charging handle).

I suspect a BCG failure of some sort. Since the carrier will not completely go forward into battery, I am unable to split the upper/lower receivers. I see no obstructions nor dirt that can cause a problem. I tried a huge blast of CLP into the chamber and on the BCG and worked it in. Didn't help though.

Any ideas appreciated. James

*Thanks a bunch for all the replies.
I just got around to getting the damn thing apart. The forward gas ring on the BCG failed and was jammed between the bolt and carrier channel. I had to tap the bolt out with a cleaning rod; it was quite stuck. The BCG gets a cleaning and/or lube before every range trip so this wasn't a maintenance issue. On this FAIL range trip I only went to 100 rounds. The rifle has just under 3k rounds through it.

Now if I can just get CMMG to send me new rings as it's still under warranty. This reinforces having spare parts and back-up on hand. James
Link Posted: 3/9/2011 4:52:17 PM EST
Remove the buffer tube then you will be able to slide the BCG out.
Link Posted: 3/9/2011 4:57:05 PM EST
[Last Edit: 3/9/2011 4:58:25 PM EST by Dano523]
Link Posted: 3/13/2011 1:55:30 PM EST
I had this problem with two different AR's I was building. I was actually building three at once. I swapped the BCG's and both then worked fine.
Neither would go into battery at all. All parts were new. I don't know why they didn't work until I swapped them. Both AR's work great though and each have had several hundred rounds through them.
Link Posted: 3/14/2011 6:49:40 AM EST
Originally Posted By Jaberie:

My CMMG M-4's bolt will not completely close and hangs up when going into battery at end of the carrier stroke. At the carrier's forward position, the bolt is jamming and is very difficult to retract (yes, that means a buttstock-ground bang with a firm grip on the charging handle).


I had this happen on a frankengun on which I had been pre-zeroing a scope using Colibri rounds in a .223 adapter. When I next tried to fire regular rounds, I saw the same symptoms you are describing. I finally tried leaving a patch soaked with lead remover in the throat overnight and then finishing up with chamber and bore brushes. That did the trick. Was it leading causing the problem? I don't know for sure, but the patches were well covered with what appeared to be dissolved lead and things were back to normal after that "treatment". My theory is that there was enough leading in the throat to prevent the round from fully chambering.
FWIW
YMMV

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