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Posted: 12/22/2012 7:25:33 PM EDT
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I have a cmmg dedicated .22 upper. When I fire one shot, the first shot fires fine, but then the bolt get stuck in the rear position, and will not slide forward again to feed the next round. The bolt, and upper is brand new. Less than 100 shots. And I have used plenty of lube, so its not running dry. The problem is obviously with the bolt carrier group. Here is a picture of it  You can see its partially pulled back, and its stuck. If it was working correctly the spring would push it into battery. It physically can go back further, but I don't want to force anything. Up until this point, it slides back and forth freely, then right at that point it is stuck, and moving it any furhter back or forth requies a lot of effort. I've also read that CMMG recommends you don't disassemble their BCG's, so I want to get your advise before I do anything. Anybody have an idea what the problem is and what the solution is? I'm going to contact CMMG, but I wanted your guys opinions first incase its an easy fix. |
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Haven't seen a problem like this before.
Two possibilities: (1) the rails may be deformed - but this would be visible from the sides. You could try gently pulling up or down on the chamber adapter and see it the bolt will slide forward when the rails flex. (2) the chamber adapter may be undersized where the rails snap in on either side or the rails may be out of spec there. I checked one of my CMMG kits without the chamber adapter and springs. The bolt will not freely slide back and forth on the rails because the rails bend just a skoosh inward. With the chamber adapter in place - the rails are spread out just enough to allow the bolt to slide back and forth easily. I believe the rails bend inward to keep pressure on the chamber adapter to keep it from slipping out from between the rails. If there were no inward pressure on the rails the adapter could not be snapped securely into place between the rails. Note in the pictures: Without the chamber adapter the bolt will not slide down on it's own. With the chamber adapter the bolt slides up and down freely:  
In case anyone is wondering if I posted the pictures the wrong way - it's a left-hand bolt.  I'm right handed and I...Awww - it's too long to explain now. Just say the left hand upper was a gift to a friend and this LH conversion is for spare parts.
YMMV |
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One other thing.....the pin that retains the extractor...might be touching the rail. This is normally the case with roll pins, but soething worth checking.
Another thing I do is 'test' the rails and slots individually......just take the bolt 'slot' and slip it in on the rail on the 'outside' and see if it moves freely. I do this to trouble shoot my assemblies, if I run into any problems. Tim |
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