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TGibe

CMMG Guard tuning (9mm)

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.So I was bored and I was looking over my 9mm PCC and I noticed the alum cases had left some marks on the barrel locking lug face. As soon as the case cleared the inner area of the barrel extension you can see the drag marks across the rear face of the barrel extension. The Ejection port was slightly to the rear of the barrel extension forming a sharp edge for case to catch on and knock it loose from extractor. My Theory is the alum cases are lighter and therefore accelerate faster and didn't clear the ridge like brass does. See pics of modified port. 

P1A.jpg

P3A.jpg

P2A.jpg

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Posted (edited)

Have occasional failures to eject with heavy springs and very light loads in my Guard upper; was planning to clearance the hell out of my port just like you have.

 

I don’t run a door, so I’m opening it up big time. Particularly since the barrel is currently out of the upper. Keep us posted.

 

 

Edited by MemphisMechanic

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So I ran it today, did the port relief solve the problem? NO! This is running Federal Alum 115gr. Glock 33 rounder, a FTE's on two rounds last eight rounds in the mag. So I inspected the spent casings and the extractor had dug some gashes on the inner edge of the case rim right at the point where extractor ends. So I polished sharp edges back and dropped a round in the chamber and pushed the carrier by hand until the extractor snapped over the rim. Then I slowly pull the carrier to the rear watching it pull the round out of the chamber expecting it to pop out when the round cleared the barrel extension. So what happen NOTHING! The round was wedged at about a 15' angle between the arc on extractor face and the far side of recessed bolt face. It was struck there firmly and it was repeatable. So I inspected the extractor face and compared it two four other 223 extractors on hand. About the same. So with my 4x magnifying eye piece  I put a round in the groove and rotated it like it supposed to roll out on extractor face. It bound with the edge of the extractor digging into the inside of the rim. So I rounded the ends extractor and polished the extractor face until the round rolled out with no binding. Performed the slow extraction again and the round hopped right out when it cleared the barrel extension. Loaded 3 mags with 10 each and burned right through them. Also noticed that ejection pattern was now consistent right 3 o'clock instead varying between 2 and 5.   

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Did this only happen with Aluminum casings?  I have never run anything but brass.

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Alum cases made it more  pronounced. I'm going to look into more 9mm PCC that uses a AR extractors, also going to compare cases rim groove shapes, also came across a site that said the ejector pin should have slight radius to make it easier to slide across the case face as it rotates out. 

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Here is bolt with modified extractor and ejector 

b1.jpg

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Interesting! So you gave the extractor claw marginally less bite, and the gun runs more reliably.

 

It does make sense that removing a spot where the case can bind up will be helpful. Thanks for sharing your discovery.

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9 hours ago, cking said:

Here is bolt with modified extractor and ejector 

 

 

I can see your problem is that your bolt is too clean.  No wonder you are having problems! 😁

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7 hours ago, MemphisMechanic said:

Interesting! So you gave the extractor claw marginally less bite, and the gun runs more reliably.

 

It does make sense that removing a spot where the case can bind up will be helpful. Thanks for sharing your discovery.

No I was very careful to not shorten hook depth, the CMMG extractor had not been head blasted and showed the cutter marks. It also had large flat section right above the edge. So mainly changed the radius and got rid of the ridges also radius the ejector.

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I give up! Today I  went out thinking everything would be fine since my test with three mags loaded with ten rounds, it cleared them all. Today I went out with three full mags didn't run single one, this is federal alum cases dumping rounds as fast as I can pull the trigger. Ejection is very consistent angle and distance till you find a mangled case sitting on top of half loaded round. Guess I try to get video

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Posted (edited)

@cking what are you running as far as buffer and spring? I dont have any issues with the factory setup; when I slowed the bolt down to flatten the gun out with a heavier spring and buffer? That’s when my problem appeared. I think.

 

Same issue you’re having: an empty case didn’t clear the port and is smashed and stuck in front of the bolt while it strips (and otherwise successfully feeds) the next round.

 

Edited by MemphisMechanic

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Also another thing to check, since I’ve had mine apart getting ready to cerakote it. The chamber is extremly short. See if your current load plunks and spins.

 

The 125gr BBI that I usually shoot it very short-chamber friendly. It’ll run at around 1.125” in a CZ, for example.

 

I had to shorten it up to 1.095” for this barrel. We’ll see if the helps... but I also enlarged the ejection port massively, so I’ll never be certain which ones was the fix. Assuming it doesnt still malfunction.

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SUMMARY: I bought a lower, barrel, bolt from CMMG. Used an existing upper and opened up the ejection port. The gun ran fine for about 1200 rounds, then it started loosing ejected case velocity, they just dribble out, then started FTEing. Contacted CMMG and sent picks of bolt and they sent me a new modified extractor. Also reminded me to use 308 carbine spring and standard carbine buffer. In mean time I started using 9mm Federal Alum ammo $$.  I found the ejector spring was weak and same extractor, both are now CS springs. When new I was running flat wire 223 spring and hyd buffer. So my hopes that this gun would be FTE free which to me means 1000+ rounds of new factory ammo with no problems.

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Tthinking about CMMG and it always extracts but some times doesn't eject. I had bolt out and was examining again the bolt and the round interaction. Found there was still spot were the round would hang binding between the extractor hook and the recessed bolt walls. Inspecting ejected cases showed that ends of extractor hook had scored the rim. So the pistols and some PCC bolts use a extractor like on the 1911. So I narrowed the extractor, but still some binding, then I polished the bolt walls. That made the difference there is NO position that case will hang, it always falls. So I waste some more ammo  testing.

BOLT3MOD.jpg

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FWIW I only shoot brass and use Eezox, a dry lube, for my CLP.  I have a KynSHOT light hydraulic buffer and a flat wound carbine spring.  I am getting their regular hydraulic buffer for comparison on the Guard and my last Colt type AR9 (which will need a spacer).  I built with the CMMG barrel, bolt and lower and a BCM clearanced upper.  I shoot off-the-shelf Fiocchi 115g.  I tried an MPX at a carbine class and to me, there was not a noticeable difference in shooting, except for our choice in triggers.

 

I have had none of the experiences you have had with your Guard.  I took the 3 day carbine class at TPC, about 500 rounds a day for 3 days without a malfunction.  I cleaned and lubed each night after class.

 

I say this just for added information to those that are thinking of getting a Guard.  If I build any more PCCs, so far 7, they will be Guard based.

 

The information in this thread is good to know if I start to have any of the problems you experienced.

 

BTW, thank you cking for the information on the hydraulic buffer on the Guard!  It really worked well for my combination. 😀

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Posted (edited)

SUCCESS BUT NOT WITH MOD 2! Now runs with Federal 115gr ALUM

Actually mod 2 was the worst FTE's every other round, but I learned something … I was wrong on less extractor needed, just the opposite more is needed.

So mod 3 is mod2 polished extractor with a brownell CS spring with black insert and CS ejector spring and polished the walls of the bolt. I think the bolt face was hair too small.  So take the bolt apart and take fine cratex on your Dremel and polish the bolt recess and edge and smooth the extractor put brownell CS springs in.

Now I can go back to the flat wire and hyd buffer too see if things are back to normal and I can run Cheap alum ammo

Now I get back to drilling the lower for lefty mag release going right in front of trigger about the size and shape of 1911 mag release.  The reason is for the man on man match we shoot you are required to make change before you shoot the dueling popper. Just two slow too use my right hand to release the mag and then put new in. With new left side release the trigger finger will release the mag while right hand is bringing a new one up. Should save a solid half second. 

Edited by cking
add

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My solution for a lefty mag release was to buile a frankenGuard with a colt-mag lower. :D

 

Share pictures of your invention, I’m curious.

 

I’ve got the mag release off the MPX installed because it was the cheapest ambi one I could find in stock.

 

Works perfectly.

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UPDATE: I'm done with the Guard. I bought an MPX and am moving on - it seems to be running fine out of the box with a mixture of PFs from 132 to 155 and OALs from 1.100" to 1.135". no problems with any of my reloads.

 

I was finally able to get out and do some testing with another shooters mags and ammo. For some reason my ammo wouldn't feed on full mags in either gun and his ammo would feed fine on full mags in both guns and both mags. He was running the red bullets and I am running Montana golds. OAL is pretty much the same. So while I do feel my ammo is the problem I have just lost confidence in the gun - at least for now.

 

I was able to get it to run reliably with less than 36 rounds loaded. Any more than that and it has problems. So I will probably sell it and let someone else fiddle with the loads. I really like the ease of cleaning vs the MPX and the recoil.

 

Bottom line: The problem appears to be my reloads, but I have tried PFs from 132 to 155 and OALs from 1.100" to 1.135". I'm thinking it must be the bullet profile or something.

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Posted (edited)

@TGibe did you ever plunk test those?

 

The Guard barrel has a freakishly short chamber. Ammo with a 125 TC coated bullet that will plunk in a CZ at 1.115” has to be loaded at 1.090” to freely spin in the Guard’s chamber.

 

Obviously that’s highly bullet dependent;  FMJs will let you load substantially longer... so I’d be surprised if your MGs were problematic at such short OALs. But it’s something to be aware of.

 

All of my hiccups so far have turned out to be related to finding the right place to position a Colt mag in an upper designed to be fed by Glock mags, so my experience isn’t really relevant.

 

I will add that I enlarged my ejection port to something truly massive before cerakoting the gun because, hey, why not? I haven’t had an FTE since that was done.

 

This hole is effectively even larger than it looks. The edges are beveled outward, so it’s a funnel guiding cases out of the port. I wasn’t taking any chances and went full retard.

 

80F474BF-B64A-40DF-ACF4-4699382ADD96.thumb.jpeg.bc87ad95908e0323400c355199df83ed.jpeg

 

3AC6E27F-DCBB-4D7F-BE99-23494B9B7DE7.thumb.jpeg.e4faec81eed1a3d54ca87e94254fd084.jpeg

Edited by MemphisMechanic

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In hopes of saving this project I ordered a new bolt. 

What I'm thinking is too much play when bolt is locked, will measure the new next to the old.

I found using a weaker spring cured the bolt over run jam

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@MemphisMechanic  What did you fill in your forward assist hole with? JB weld?

 

 

I've got an Anderson 458 socom upper for my Guard build and I can't decide what I want to do with the f/a.

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Posted (edited)
2 hours ago, ProfessorGascan said:

@MemphisMechanic  What did you fill in your forward assist hole with? JB weld?

 

 

I've got an Anderson 458 socom upper for my Guard build and I can't decide what I want to do with the f/a.

 

I bought a forward assist plug off of eBay that is retained with a roll pin just like the factory assembly.

 

Installed it with a lot of JB weld slathered on. Cut the FA down close to the shape you see, then finished with a belt sander.

 

If I were building another gun, I’d start with a slickside upper and the Guard barrel & bolt. I didn’t know those parts were available when I ordered a complete upper. I paid more... for a handguard and forward assist I didn’t want.

 

Edited by MemphisMechanic

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Make sure you check your gas keys.  Mine was loose as a goose!

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New to the Guard/Banshee as well as PCC.  I have a brand new Banshee upper for my Spike's CalMulti lower.  The Banshee will make for 3 caliber uppers (so far) I can pop onto my one sbr registered lower.  I have not tested it yet waiting for Endomags to ship.

 

Fwiw Brian at CMMG told me that their Guard/Banshee builds are shipped with a standard carbine buffer and a .308 spring.  I'm going to give it a go when the Endomags arrive and hope to be running well without having to swap buffer/spring or install a mag block.  I may/may not have to use one of the carrier weights in the CMMG tuning kit to get things running right.

 

I haven't tried competition yet but one of the reasons I joined here is to learn more before I dive in.   Looks like I came to the right place to learn about it.

Edited by HDSledge

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Update of new BCG. CMMG put a new BCG in my Guard, it runs! However the recoil impulse is noticeable harsher even with 223 flatwire and hyd buffer. Which makes me think they changed the angle of the locking lugs to leave more momentum in the BCG. So now I'll move on to testing a 308 spring and probably a 308 flat wire.

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