CMMG Guard upper & QC10 / colt lower

[MemphisMechanic]

Well... the fool’s errand may just have succeeded!

 

I spent this afternoon running 300+ rounds through a complete CMMG Guard upper. It’s mounted onto a QC10 Colt mag lower which is fitted with a KE Arms SLT-1 trigger.

 

First obstacle: The “wings” on the feed ramp needed to be ground down. I did not touch the actual feedramp. 

 

Before and after:

 

 

 

The other issue? The mags sit too high for the 5.56-style bolt in the Guard. The magic number for my lower turned out to be .015”.

 

Started with a notch this tall:

 

Hand-filed the left one open at the top until it’ll seat on a closed bolt easily:

 

It fed fine like that, but monopodding the gun causes it to lock up. So I backed the notch with a brass block that was cut & sanded to fit the mag tube:

 

Then welded the bottom of the window with my MIG welder, filed it to fit the mag catch effortlessly, and hit it with some cold blue:

(next post)

 

 

 

 

[MemphisMechanic]

 

Fits great now, and feed flawlessly. I put 300+ round through the gun, including monopodding off the magazine without any issues.

 

”Gear teeth” on the Guard’s bolt pass perfectly through the feed lips, it’s never double-fed nor failed to strip a round:

 

Completed rifle with the $$$ handguard that’ll be mounted on it shortly:

 

I did initially try to run a gas deflector at the back of the ejection port because I’m a lefty. See all those marks on the inside corner? I had all kinds of weird failures because the empties were bouncing off of it and heading back into the chamber on my first range trip. I removed the entire dustcover assembly and it runs flawlessly so far!

 

What do the blowback guys in my group of USPSAers think? It feels soft, and funny. Not like a tuned blowback gun. Not like a gas gun either.

 

I like it. Now I have to figure out how to flatten her out: tons of dot bounce in it the way I built it!

[MemphisMechanic]

The baby steps when it comes to buffer tuning:

 

 

I have a 5.0oz Kyntech (they make the Blitzkrieg) 5.56 buffer on the way, which is an ounce lighter and 3/4” shorter than the blowback 9mm blitzkrieg unit. It should be short enough to let me actually lock the long Guard bolt back. I hope.

 

I also have blue and red sprinco carbine recoil springs on the way, which are both heavier than the factory carbine spring is. Testing to follow, before I add a comp and try to get it totally flat. ish.

 

 

Edited May 7, 2018 by MemphisMechanic

[BartCarter]

The Guard is designed to use the standard length (3.25") buffer.  The KynShot should work fine.  I used the RB5000L (light) KynShot buffer, which seems to work fine.  I don't know what the difference would be with the standard one.  The light was recommended for the 300 Blackout.

[BartCarter]

Nice job on the build.  If I were to build a Colt setup, I'd follow what you did.  Looking forward to your buffer "testing."

[MemphisMechanic]

What’s intriguing to me is that the more the buffer weighed, the flatter the gun shot. I’ve talked with three people who are running a Guard rifle... and all of them encouraged me to try to keep it light.

 

Granted, lighter buffer systems did make for a softer shooting gun. But the dot bounced like crazy. 

 

A heavier 6oz blitzkrieg buffer gave it a bit more recoil, but the dot didn’t lift off the top of an 8” plate at 15 yards anymore.

Edited May 7, 2018 by MemphisMechanic

[MemphisMechanic]

1 hour ago, BartCarter said:

The Guard is designed to use the standard length (3.25") buffer.  The KynShot should work fine.  I used the RB5000L (light) KynShot buffer, which seems to work fine.

 

Good to know on the length: The gun has an undeniable preference for a 5 - 6 oz buffer weight, at least with a factory carbine spring I’ve run thus far.

 

I ordered the 3.25” buffer meant for .307 rifles, which is 5oz in weight. A guard ships with a conventional 5oz buffer from what I can determine... so we’ll see how this goes.

 

Particularly when paired with the heavier sprinco blue and red carbine springs that are also on the way.

[Nathanb]

Is the Memphis mechanic diving into pcc?

[MemphisMechanic]

5 minutes ago, Nathanb said:

Is the Memphis mechanic diving into pcc?

 

Damn right. I have a friend who can’t shoot handguns winning HOA at locals. He needs a good stomping. And I want to learn to be truly proficient at manipulating an AR. Plan is to run this rifle for the rest of the season once it’s built.

 

Then it’s back to Production.  

[Nathanb]

It’s fun. A lot of fun but I’m trying to avoid the rabbit hole of building a dedicated gamer gun and I’m tired of changing the mpx around from the suppressed sbr to longer barrel and what not

[MemphisMechanic]

Shooting a division without retooling the gun for it? Man that’s a hard patch to hoe!

 

Good luck resisting that temptation.

[bmiller]

4 hours ago, MemphisMechanic said:

 

 

 

Then it’s back to Production.  

 

The PCC addiction is pretty hard to beat.  I said that about open a while back.  I think my comp is full of spider webs.

[Bullet56A]

Is that  Kiana Reeves in the video?

 

 

[Steve RA]

Looks a little heavy for "John Wick" !     

[rowdyb]

You always have a cool build going on, I like it. (And my pcc build finished today as well!)

[Nathanb]

You need a laser. Lol

[MemphisMechanic]

3 hours ago, rowdyb said:

You always have a cool build going on, I like it. (And my pcc build finished today as well!)

 

I never can build something the same way that everyone else has done it!

 

 The Guard platform looked interesting, and I knew I wanted to run Colt mags - mostly because I’m a lefty... but also because I prefer the “stab it straight in” reload, and double-column feeding arrangment.

 

Edited May 8, 2018 by MemphisMechanic

[Norther]

This is interesting. Need to think about it...

 

Could you file the 0.015” off the top of the mag catch? To avoid modifying magazines. I guess you’d still have the monopodding issue without welding them. 

 

Too many magazines to modify, and it would be good if they’d still work in other guns. 

[MemphisMechanic]

5 hours ago, six-gun shooter said:

This is interesting. Need to think about it...

 

Could you file the 0.015” off the top of the mag catch? To avoid modifying magazines. I guess you’d still have the monopodding issue without welding them. 

 

Too many magazines to modify, and it would be good if they’d still work in other guns. 

 

That will definitely work. I have three mags, and only welded one notch back down. Waited to do the others until I had live-fire tested that idea.

 

As long as you don’t monopod, the wider mag openings work absolutely flawlessly. So you don’t HAVE to weld them if you don’t want to.

Edited May 8, 2018 by MemphisMechanic

[mike4045]

I have been running uzi mags in my gun for almost 2 seasons. 

[MemphisMechanic]

1 hour ago, mike4045 said:

I have been running uzi mags in my gun for almost 2 seasons. 

 

What gun? 

[mike4045]

I have a Hahn top loading block in a factory lower. Upper was built by 10 ring precision. Uzi is a tougher mag than the metalform colt mags.

 

[MemphisMechanic]

All right guys. I did some recoil system / compensator testing and tuning yesterday:

 

 

The winner turned out to be a ported barrel, stronger Sprinco recoil spring (blue), the kynshot 5oz .308 buffer, and a modified surefire .223 comp. See the 25 second timestamp in the video.

 

I tried the factory cabine spring again to verify, and sure enough the muzzle began to flip again.

 

With the blue spring? Nice and flat, and when I didn’t overcompensate and drive the gun off-center, it chewed a 2” hole in the target shooting pairs at 12 yards.

 

Edited May 15, 2018 by MemphisMechanic

[MemphisMechanic]

I went a little nuts on the gun before I headed to the range. Since I’m sending the barrel out to get cut down to 14.5” for a pin & weld, I decided to try something in the portion that will be cut off anway. A 5/32” port in the barrel:

 

 

Then I bored the comp out with a 3/8” drill bit. Given the pilot hole, a drill press with a good square table made this easy to keep concentric without a lathe:

 

 

 

Bored out the top ports in the comp to 5/32” as well:

 

 

Finished product:

 

 

I swapped the A2 back onto the gun before going to the range, so that meant I got to make the people in the lanes next to me cringe at my barrel vise when it was time to install the compensator:  

 

[Flatland Shooter]

On 5/15/2018 at 9:46 AM, MemphisMechanic said:

I went a little nuts on the gun before I headed to the range. Since I’m sending the barrel out to get cut down to 14.5” for a pin & weld, I decided to try something in the portion that will be cut off anway. A 5/32” port in the barrel:

 

Then I bored the comp out with a 3/8” drill bit. Given the pilot hole, a drill press with a good square table made this easy to keep concentric without a lathe:

 

Bored out the top ports in the comp to 5/32” as well:

 

 

Finished product:

 

 

 

 

 

 

Came across this while thinking of getting some holes put in a "test" barrel.

 

How did the popple hole and new comp work?  Something you will try again?