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Page AR-15 » Rimfire and Pistol Calibers
AR Sponsor: bravocompany
Posted: 10/1/2017 7:52:28 PM EDT
I've got a cmmg 8.5” Mk9 9mm pistol that I'm having a problem with. It's locking the bolt carrier group lock to the rear about 10 % of the time. I'm running metalform mags and it's doing it with all of them.
I'm shooting 124 grain fmj rounds that I'm reloading myself and is what I'll be using for training with this pistol.
I'm also using a stock carbine spring with a PSA 9mm red tipped buffer which weighs 5.6 ounces.
Any suggestions?
Thanks
Link Posted: 10/2/2017 6:41:00 PM EDT
[#1]
If it's a complete MK9 then it should have came with a cmmg  buffer and an ASC  magazine.  Try the ASC magazine. I have a complete mk9 PDW, like yours,  and have had a couple issues like you are describing with the metalform mags but not the ASC mags.  Make sure you clean the mags very well or they can stick a little and cause problems too. If that doesn't work,  call CMMG, they are very easy to work with.
Link Posted: 10/2/2017 7:25:26 PM EDT
[#2]
Thanks for the advice. I did forget to mention that it did happen with the ASC magazines as well. The mags were cleaned and lubed.
Link Posted: 10/2/2017 7:38:54 PM EDT
[#3]
So what you are saying is, The bolt is not locking back after the last round in the magazine is fired. It could be your reloads are powerful enough to push the bolt back enough to strip a round from the Magazine, but not powerful enough to push the bolt back enough to engage the Bolt stop. To confirm this, try shooting some Factory loads.

Also something else to try, With an empty Magazine inserted, Cycle the action by hand, Does the Bolt lock back? I once had a Bolt Stop with elongated finger on My Colt Magazine lower that was just a shade too short and was not engaging the Magazine follower.
Link Posted: 10/2/2017 8:34:07 PM EDT
[#4]
No that is not what I’m saying. It is locking back at will in the middle of a magazine at any time. It always locks back after the last round.
Thanks
Link Posted: 10/2/2017 9:12:20 PM EDT
[#5]
I know what your saying the problem is now, I had the same Problem on one of my Colt Magazine Lowers, The finger on the Bolt Stop was being tripped every so often by the rim of a round in the magazine, I trimmed it ever so slightly so the Rounds didn't engage the Bolt Stop but the follower did.
Link Posted: 10/16/2017 9:37:32 AM EDT
[#6]
Update: problem fixed.
Upon disassembly I noticed the bolt catch was moving freely with no spring tension. I took the bolt catch off and noticed the detent and spring were jammed inside the hole and not providing any tension on the bolt catch. Took it out, cleaned up the hole, installed a new spring and detent ( just in case) and the problem stopped.
I hope this might be able to help somebody else that has this problem. Thanks to everyone for their suggestions.
Page AR-15 » Rimfire and Pistol Calibers
AR Sponsor: bravocompany
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