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Anyone tried the new CMMG Guard


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15 hours ago, bmiller said:

On the NFA lower, over insertion is stopped by the ejector, it acts as a stop for the mag.  You have to remove the ejector from the lower to run the guard bolt because it is not slotted.  I have been machining stops that drop in place of the ejector of the NFA lower.  My NFA lower works great the the guard upper with the stop installed.

Maybe my Spartan lower is not an NFA then because while the ejector will eventually stop the mag it is high enough where I can go past the mag release a good distance if I hold the mag release down. No biggie either way was just trying to clarify the CMMG lower mag stop...

 

thx,

Allen

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1 hour ago, TGibe said:

Maybe my Spartan lower is not an NFA then because while the ejector will eventually stop the mag it is high enough where I can go past the mag release a good distance if I hold the mag release down. No biggie either way was just trying to clarify the CMMG lower mag stop...

 

thx,

Allen

Ok. Once again I’m wrong... the ejector does stop the mag from over insertion on my NFA. Sorry for the confusion, but sometimes I can be dense...

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A friend and I tested a few guns yesterday.  We shot some slo-mo video of him shooting my pcc with the guard system installed.  Those loads are 144 power factor.  124 grain bullets.  In the next couple weeks that upper is going to get chopped, popple holes drilled, and a bloop tube pinned and welded. The lower is my billet PSA with my magstop installed.  So far, it has ran flawless.

 

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Finally managed to get some time off work so I had both guns down at the range today.

 

Thoughts-

The delayed blowback is very soft. The cycling is still chunkier than a 5.56 AR but much less than a blowback. In particular I noticed the lack of the smack at the end of the cycle when the blowback bolt closes.

It seems to like different loads than the blowback gun. I've been using a heavy bullet in the blowback gun (a 165gr load) but it felt like it took forever to cycle the guard. My doubletapz were closer together with a snappier load.

 

Anyway, a little video

 

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1. All I run is hiperfire in everything.  I know it works fine with mine.  

2. dandbmiller@hotmail.com

3. I put the over travel stop in, and had to slightly file some mags for clearance for the bottom lug.  That’s all I did with mine.  It is the softest shooting setup I have fired.

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I shot an indoor match yesterday, after the match we shot my two pccs next to a mpx all with my ammo.  My blowback gun was the lightest weight by far, and the sharpest recoiling.  I just have a ninjaflex spacer in the buffer tube. I am running a PSA 9mm buffer and spring.  I am not running any type of pogo stick contraption in it.  I might try one in the future.  The mpx was a  stock carbine,  I am not sure what it weighs.  My rifle with the guard upper weighs about 7.5 pounds.  A bunch of guys shot them.  The overall opinion was the sig was the softest, but not by very much.  The guard was a close second. They thought my blowback gun recoiled just slightly less than an elephant rifle after shooting the other two.  Most shooters thought the sig had a chubby, bloated feel, but they thought it was very smooth.  The ammo was 124 grain 144 pf out of a 16” barrel.  The same load was used in all rifles.  I am not sure how they did it, but two shooters managed to get my guard to jam. Something I had not had happen to me.  Both shooter said they were holding it loosely. Both are pretty strong guys, and the one guy, if he wrestled a grizzly, I am not sure who I would bet on.  Not sure what was going on there.

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There are some limitations as you guys are seeing.

 

The Guard lower puts the mag about 0.100-0.125in lower, it also has a slightly different angle. 

Barrel is a normal M4 extension but with center receiving lug ground down.

Bolt is a custom with the same lug pattern, bottom 2 lugs cut down to clear feed lips. 

 

@bmiller has done what I thought of, to machine and install a Mag travel stop in the NFA lowers however for proper mag placement you have to lower the mags. I would file off the mag catch not the actual mags. 

 

I have the Guard lower and doing some testing of my own, the system is a step in the right direction but not quite there yet. 

Edited by Marshal82
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I did not need to alter the mags other than some Glock mags need opened at the top a small amount. Mine does not like ets mags, they are too narrow between the feed lips.  I keep thinking about a piston drive system for system.  I like the mpx,  not sure it is for me.

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On 11/20/2017 at 4:29 PM, mahamoti said:

... CMMG says uppers "may require modification." ...

Revisiting this, I feel that an AR15 upper needs modification.  I took my new build from the CMMG Guard BCG/barrel/lower for sighting in and testing of magazines, etc. I was getting about 1 in 15 failing to eject. On another forum that has a long Guard thread, CMMG said if you don’t use their upper, you may have to clearance the ejection port.  I didn’t think much of this as Colt SMG uses a standard ejection port that works well with 9 mm.  Now I feel differently.

 

In comparing the factory Guard and my AR15 upper, there is quite a difference between the Guard ejection port and an AR 15 ejection port.  The guard ejection port is clearanced on the lower edge all the way to the bolt bearing surface.  The top edge is clearanced on the first half of the port to the extent it takes out about half of the upper bolt bearing surface.  The ends of the Guard port are also more squared off instead of round, probably due to the end mill size.

 

I feel CMMG did this for reliability and maybe to accommodate different types of ammo.  Also, who knows the differences in the shell ejection between a Colt SMG style ejector and an AR15 type of ejector.

 

I had my mill set up for milling lines in my last linear comp.  Should not be hard to do an exact reproduction of the Guard upper.  If this stops the ejection problem, it would make sense to use the Guard upper with the BCG/barrel/lower for a custom build.  (If you don’t have a mill with a rotary table.  :P)

 

 

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  • 2 weeks later...
On 1/17/2018 at 4:47 PM, BartCarter said:

Revisiting this, I feel that an AR15 upper needs modification.  I took my new build from the CMMG Guard BCG/barrel/lower for sighting in and testing of magazines, etc. I was getting about 1 in 15 failing to eject. On another forum that has a long Guard thread, CMMG said if you don’t use their upper, you may have to clearance the ejection port.  I didn’t think much of this as Colt SMG uses a standard ejection port that works well with 9 mm.  Now I feel differently...

 

 

Finished milling my upper to open up the ejection port.  I milled the top front half 0.038" and the entire length of the bottom 0.110".  I ran all my Glock mags from fully loaded and not one failure to eject.  Today I ran a steel match and not one failure to eject.  400 rounds total.

So, if you are having any ejection problems with your Guard barrel/BCG build, I highly recommend clearancing the ejection port.

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