CMMG Guard tuning (9mm)

[BartCarter]

On 9/25/2018 at 11:09 PM, cking said:

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.

 

Did you get to test this out yet?  I had my first failure to eject in a match this weekend.  I have at least 6,000 rounds through my Guard.  I am going to disassemble my bolt and give it a good cleaning.  I admit that I have not disassembled the extractor or ejector for cleaning before, hope that is all it is.

[Acer2428]

Does anyone have the specs of the ejection port of an actual Guard upper? I was using a .458 upper, but I'm building a 'red' one for my wife and using an anodized 223 upper and want to dremel/file out as little as possible. Seems like .458 spec up front and then just some light removal of the bottom on the rear 1/2 and nothing up top?

[MemphisMechanic]

6 hours ago, Acer2428 said:

Does anyone have the specs of the ejection port of an actual Guard upper? I was using a .458 upper, but I'm building a 'red' one for my wife and using an anodized 223 upper and want to dremel/file out as little as possible. Seems like .458 spec up front and then just some light removal of the bottom on the rear 1/2 and nothing up top?

 

Here’s mine. I just opened up the front half of the port as wide as makes any kind of sense. Then a pinch more

 

458 socom port? Nah. Keep going.

 

(Click the photo and swipe for a better view)

 

 

Edited October 16, 2018 by MemphisMechanic

[Acer2428]

I saw yours in my search and it looks great. My only concern is I'm not refinishing once it's done. Does the rear of the ejection port actually matter? I've never really looked to see where the brass/base is when it's ejected. I only mentioned .458 since that's what I was using and it had so far worked 100%.

[BartCarter]

22 hours ago, Acer2428 said:

Does anyone have the specs of the ejection port of an actual Guard upper? I was using a .458 upper, but I'm building a 'red' one for my wife and using an anodized 223 upper and want to dremel/file out as little as possible. Seems like .458 spec up front and then just some light removal of the bottom on the rear 1/2 and nothing up top?

 

I milled the top front half 0.038" and the entire length of the bottom 0.110".

[BartCarter]

On 10/15/2018 at 10:37 AM, BartCarter said:

 

...  I had my first failure to eject in a match this weekend.  I have at least 6,000 rounds through my Guard.  I am going to disassemble my bolt and give it a good cleaning.  I admit that I have not disassembled the extractor or ejector for cleaning before, hope that is all it is.

 

Well, shame on me.  My bolt really needed a thorough cleaning.  Ran a match last night and it was flawless.  So, more bolt cleaning for me.  

 

Also, I did call CMMG to see if they had advice on this.  They are sending me their updated ejector and buffer springs.  I asked and the new buffer spring for 9mm Guard is now a .556 carbine spring. 

[tha1000]

 

@MemphisMechanic  that build is nuts.  I like the crab trap that you turned into a hand guard.

[MemphisMechanic]

15 minutes ago, tha1000 said:

 

@MemphisMechanic  that build is nuts.  I like the crab trap that you turned into a hand guard.

 

That’s about $300 worth of crab trap from Brigand arms. Including the barrel nut, it’s just over a quarter of a pound. Between that and having the barrel turned down... now I pick up everyone else’s rifle and think it’s a heavy pig up front.

[tha1000]

Just now, MemphisMechanic said:

 

That’s about $300 worth of crab trap from Brigand arms. Including the barrel nut, it’s just over a quarter of a pound. Between that and having the barrel turned down... now I pick up everyone else’s rifle and think it’s a heavy pig up front.

 

I bet.  

[JsK]

10 hours ago, MemphisMechanic said:

 

That’s about $300 worth of crab trap from Brigand arms. Including the barrel nut, it’s just over a quarter of a pound. Between that and having the barrel turned down... now I pick up everyone else’s rifle and think it’s a heavy pig up front.

 

One of our local shops has a 8" guard barrel and bolt, toyed with the idea of building a shrouded barrel gun if I could get it pinned and welded at a reasonable cost. How much different are the CMMG lowers and upper from the normal AR9 parts?  Its not nearly as intersting if I have to use there upper and lower.

[MemphisMechanic]

 

@JsK you’ll need some sort of stop to prevent Glock mags from overinserting since you won’t be running an ejector in the lower.

 

Any upper will work so long as you open up the ejection port.

 

My gun feeds from Colt mags, which took a lot more work to get flawless reliability from; CMMG never intended the Guard to feed from those mags.

 

Edited October 19, 2018 by MemphisMechanic

[JsK]

I guess my question at this point is if you thinks its worth the trouble vs just building a straight blowback gun with a ULW barrel.

How much softer does a guard shoot?

[MemphisMechanic]

It’s noticeable.

 

The primary benefit so far has been that it’s gentler on triggers... and it does not use the AR9 style firing pin which is failing frequently in every other application. JP included.

 

Be aware that your bolt is longer, and the Guard can’t use buffers like the Blitzkrieg designed for the AR9 with having a super short stroke.

 

Doing it over, I’d run an A5 buffer tube. You can always add spacers to shorten the length, but you can’t add length to a conventional carbine tube.

 

[Acer2428]

7 hours ago, MemphisMechanic said:

Guard can’t use buffers like the Blitzkrieg designed for the AR9

 

Wait, you mean you can't use it AND short-stroke it? I am currently running the AR-9 buffer in my guard setup since that was the lightest recoiling. It has worked flawlessly despite being both longer and heavier than CMMG recommends. 

 

Quote

I guess my question at this point is if you thinks its worth the trouble vs just building a straight blowback gun with a ULW barrel.

How much softer does a guard shoot?

 

It's noticeable if you shoot back to back, but honestly it's not going to win or lose you any matches. It's also FAR more expensive at this point. I did it because I like projects and to tinker. In the end, it's a half-step between a BB AR-9 and an MPX. If you're heavily invested in Glock mags, mag pouches, upper/lower, etc. and all you need is the bbl/bolt combo, go for it. 

8 hours ago, JsK said:

How much different are the CMMG lowers and upper from the normal AR9 parts?

 

Not much, but enough that MOST people will tell you they wish they had just bought a Guard to begin with. Buy a .458 type upper (I used a Spartan/NFA from JoeBob) and an NFA/Spartan lower. The mag height is different, so I had to file down the mag catch so bolt could retract/lock over the follower of an empty mag. I used the stock/plastic piece for proof of concept, then filed on the Odin one. Works fine. I also took a razor to some of the sharp edges on the nose of the follower.  You also need something to prevent over-insertion, which I bought from BMiller on here. 

Edited October 19, 2018 by Acer2428

[MemphisMechanic]

Of course you can short stroke it. I do. It’s advantageous.

 

But in a carbine tube with the longer 5.56-style bolt, you are required to short stroke by default if you run anything like the Blitzkrieg buffer meant for a conventional blowback AR9.

 

Running the A5 tube gives you more length to play with spacers AND wave spring and whatever else you choose.

Edited October 19, 2018 by MemphisMechanic

[HDSledge]

8 hours ago, MemphisMechanic said:

Be aware that your bolt is longer, and the Guard can’t use buffers like the Blitzkrieg designed for the AR9 with having a super short stroke.

I have a Guard upper/Spike's ST-15 Cal-Multi lower and would like to try a Kynshot RB5004  AR10 hydraulic buffer in my LWRC UCIW (6" tube) stock with its' flat spring.  the RB5004 is 2.5" long or .25" longer than the UCIW buffer.   It is .6 oz heavier at 3.7 oz.

 

I am very new to the 9 mm PCC world and still learning.  Do you think I may have success with the Kynshot hydraulic buffer with my setup?  Kyntech replied to my inquiry and said it shoulk work.  I have the CMMG tuning kit in case I need to add carrier weight.

 

The members here have PCC knowledge far beyond mine so your opinions are welcome and appreciated.  Concerns?  Warnings?

[BartCarter]

Kynshot hydraulic buffers come in different offerings, light to heavy.  Bryan at NoKick works directly with Kynshot.  He recommended the 5005 (for a 308) for my Guard, but does not have hands-on experience with the Guard.  He said that a 9mm needs a more robust buffer, even the Guard.  I had a light, (5000L) which I sent back (hydraulic leak) and still have a .556 (5000).  I'll probably test the 5005 when it arrives and the 5000  to see the differences.  If I like the 5005 most, I'll put the 5000 in one of my .556's.

 

I have been using carbine buffer springs and they seem to be fine, but was told that I might need to go to a 308 buffer spring.  CMMG now uses a carbine spring in their 9mm Guards.  I will probably test all combinations to see which one has less muzzle movement and go with that combination.  After all, I don't find any 9mm recoil a problem.  The last couple of matches I have been using the CMMG buffer and it seems to be just fine.

 

I know the Blitzkrieg was designed specifically for 9mm blowback ARs, but no LRBHO with a Guard unless you go with an A5 buffer tube.  That's why I am going with the normal Kynshot carbine lengths.

[HDSledge]

Bryan at CMMG told me they use a .308 spring and standard carbine buffer in the Guard.  Is that still the case?

[BartCarter]

3 minutes ago, HDSledge said:

Bryan at CMMG told me they use a .308 spring and standard carbine buffer in the Guard.  Is that still the case?

 

As of Monday, the 15th, Bryan told me the upgraded spring he is sending me is the carbine spring, not the .308 spring.

[HDSledge]

Good to know.  I wonder why they switched?

[BartCarter]

Well, my thought has always been that a spring should handle the buffer movement and have enough recoil to reliably chamber the next round.  The carbine springs have always done that for me.

[cking]

I've wondered why some many different opinions on felt recoil of the Guard. My first bolt was extremely soft shooting but failed after some 1500 rounds.  The new bolt is far from soft and works well with a flat wire 308 and hyd buffer but it slings the brass. I found some close shots of the first bolt will have take some pics of the new version and see if we can any difference. Been busy rebuilding my surface pro 4 MSFT gave me a new one due to screen flicker heat problem.

[cking]

Shot mine today at a steel match, not happy way to much recoil, going to try adding carrier weights.

[cking]

If you go CMMG https://www.cmmginc.com/product/action-tuning-kit-mkg/#  it's says 3.4 oz standard for 9mm DOES anybody carrier come with the weight?

[HDSledge]

8 hours ago, cking said:

If you go CMMG https://www.cmmginc.com/product/action-tuning-kit-mkg/#  it's says 3.4 oz standard for 9mm DOES anybody carrier come with the weight?

If your question is does any carrier weight come installed?  The answer is no.

 

I have the tuning kit and am going to try a Tubb Flat Spring with Kynshot RB5005 AR10 hydraulic buffer (5 oz.)  if my stock Spike's T3 (5,4 oz) buffer and spring doesn't feel right.  Plus I can use the tuning kit weights to tune more.  Took off my LWRC UCIW since tube was too short to allow much tuning of springs/buffers and put back on the Magpul CTR (7" tube).

 

New to all this and don't compete yet.  Just working on getting to the smoothest most reliable operation.

Edited October 24, 2018 by HDSledge