My CMMG 9mm AR carbine is teh suxors

jimbobborg · May 23, 2018

It is currently sitting in my trunk, with a dead round in the chamber. I can't get it to go through more than one magazine without malfunctioning. I've swapped out springs, buffers, and triggers, it still jams the spent case sideways into the chamber. At this point, I think the lower receiver is out of spec. When I run the complete upper on another lower, it's like butter. The Spikes 9mm AR uses the same magazines, and ran flawlessly yesterday. I'm trying one more thing with this lower, then I'm tearing it apart and getting another dedicated 9mm lower. I'm going to use my known good upper on it. If it continues to flake, it's going to be a paperweight. Or I'll send it to my smith, wait two months and spend $$$ on something that should be right the first time.

Wynter · May 23, 2018

I would check the buffer first. My friend had a similar issue and got the heaviest buffer that he could. That fixed his issue. Your results may vary.

jimbobborg · May 24, 2018

I have an 8 oz buffer in it.

John from MD · May 24, 2018

Can you explain the malfunction a little better? Jamming the case sideways in the chamber does not compute. Do you mean that the case is in the receiver sideways and is hit by the BCD? Can you duplicate the problem by cycling the action by hand?

engineerbrian · May 24, 2018

jimbobborg said:
It is currently sitting in my trunk, with a dead round in the chamber. I can't get it to go through more than one magazine without malfunctioning. I've swapped out springs, buffers, and triggers, it still jams the spent case sideways into the chamber. At this point, I think the lower receiver is out of spec. When I run the complete upper on another lower, it's like butter. The Spikes 9mm AR uses the same magazines, and ran flawlessly yesterday. I'm trying one more thing with this lower, then I'm tearing it apart and getting another dedicated 9mm lower. I'm going to use my known good upper on it. If it continues to flake, it's going to be a paperweight. Or I'll send it to my smith, wait two months and spend $$$ on something that should be right the first time.

I've had issues with my CMMG 9mm AR. They have really good CS and stand behind their product, so just send it back to them and let them deal with it.

To date, CMMG has replaced my charging handle, magazine, upper receiver and BCG :rolleyes:

erwos · May 24, 2018

Is this a Mk9 or a MkG?

outrider58 · May 24, 2018

Is this the same gun that has always vexed you?

Was the receiver extension face ever trued?

Only other thing I can think of is maybe too much buffer weight, but if this is THAT gun, it seems like you've tried just about everything and anything.

Too much or too little bolt speed can both cause the same kind of problems.

Brownl345 · May 25, 2018

It sucks, OP, that you've had these problems. My MK9LE has been 100% problem free. I'm currently trying to sell it for other reasons, but more likely, I'm going to keep it and SBR it.

jimbobborg · May 25, 2018

Yes, this is the troubled AR that has vexed me for almost a year now. I didn't buy this as a complete rifle from CMMG, it has a CMMG complete 9mm lower, and I put a CMMG 9mm upper receiver on it, with a Wilson barrel, CMMG bolt, and a few other bits and pieces.

To continue my story, I took this and my Spike's Tactical 9mm AR to Sharpshooters for testing. I swapped uppers after I got some rounds down range. When I put the CMMG upper on the Spike's lower, it failed to eject. When I put the Spike's upper on the CMMG lower, it worked. So I talked to Tim from Sharpshooters after I finished off a few more magazines. He suggested that I get the chamber checked on the CMMG. The cartridge may be going in too deep, which prevents the extractor from going over the rim of the cartridge.

BenL · May 25, 2018

Sorry to hear about your continuing issues. Nothing's more aggravating than a 9mm AR that won't run.

For mine, it's magazines. I have a handful that will work reliably and a bunch that won't.

jimbobborg · May 25, 2018

BenL said:
Sorry to hear about your continuing issues. Nothing's more aggravating than a 9mm AR that won't run.

For mine, it's magazines. I have a handful that will work reliably and a bunch that won't.

The truly weird part about this is before all of the malfunctions, all of the magazines worked. Then some would cause problems. Now all of them are causing problems. All of the magazines work with the Spike's. Even the three crappy ProMags I picked up for $10. I think I'm going to have my smith go through it. At least now I have a fully functional 9mm AR with a spiffy new JP trigger group.

jimbobborg · May 28, 2018

Someone on another site ( http://www.m4carbine.net/archive/index.php/t-16305.html ) mentioned that the extractor may cause problems, especially if you're shooting cheap ammo or lead bullets. So I compared the two BCGs that aren't working to the one that is working. I used an empty FC case I found on the floor of my room. The two BCGs that were causing problems were hard to get the rim over. The working one had no problem. How do I clean out behind the extractor?

My CMMG 9mm AR carbine is teh suxors

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