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TGibe

CMMG Guard tuning (9mm)

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I'm looking to use my lower power factor loads (124gr projectiles at 1100fps) but it isn't cycling the bolt well enough to eject rounds reliably). Cycling seems reliable at 1200fps... I would really like a 130-140PF load to run reliably...

 

What are USPSA shooters doing to the Guard 9mm to allow them to run reliably at lower PFs?

Has anyone replaced the stock buffer and spring in the 9mm guard?

Or are folks just running hotter loads?

 

Thanks much!

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Your 1100 FPS loads are at 136 PF, and should be just fine. 1015 FPS is your threshold for that weight bullet. 

I personally know very little about the Guard. According to what I have read it utilizes a delayed blowback that slows the bolt/buffer as it comes out of battery which in turn allows the use of a lighter bolt/buffer assembly.

My opinion is that the good folks at CMMG have got the bolt/buffer mass (weight) correct. That being the case I would suggest you try out a couple of different springs and see what works best. 

I would suggest you visit Blitzkrieg Components website and check out their selection of buffer springs. Start with the softest and work your way up, or at least get one that is weaker than the one you have. It's my guess that it's a bit too stiff.

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34 minutes ago, MikieM said:

Your 1100 FPS loads are at 136 PF, and should be just fine. 1015 FPS is your threshold for that weight bullet. 

I personally know very little about the Guard. According to what I have read it utilizes a delayed blowback that slows the bolt/buffer as it comes out of battery which in turn allows the use of a lighter bolt/buffer assembly.

My opinion is that the good folks at CMMG have got the bolt/buffer mass (weight) correct. That being the case I would suggest you try out a couple of different springs and see what works best. 

I would suggest you visit Blitzkrieg Components website and check out their selection of buffer springs. Start with the softest and work your way up, or at least get one that is weaker than the one you have. It's my guess that it's a bit too stiff.

Thx. I think they have it perfect for normal commercial ammo. I have some springs and weights to try out and am hoping to document what owners have done to get it tuned for minor PF loads.

 

Cheers!

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I am told the Guard needs at least 500 rounds through it - presumably to break in the locking lug-barrel extension interface. Run 500 of a hotter load, then circle back and see if it will run the lighter stuff.

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Or see if you can polish the snot out of the bolt, carrier, and barrel’s locking lugs. That goes a long way toward making ligther ammo cycle things more consistently

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Mine likes normal power stuff, like 135-140pf (out of a handgun). When I use my powderpuff loads it doesn't run as well. IMO the snappy loads are less dot disruption too.

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1 hour ago, NickBlasta said:

Mine likes normal power stuff, like 135-140pf (out of a handgun). When I use my powderpuff loads it doesn't run as well. IMO the snappy loads are less dot disruption too.

 

Thanks Nick. I developed this load for my blowback PCC and when I chrono'd it in a G17 it barely made 125PF.  LOL

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7 hours ago, StealthyBlagga said:

I am told the Guard needs at least 500 rounds through it - presumably to break in the locking lug-barrel extension interface. Run 500 of a hotter load, then circle back and see if it will run the lighter stuff.

I had heard this and forgotten. I will put some more rounds through it and test again. Thx.

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Are you running any of the weights in the carrier? If so start by running with that removed. 

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The bolt lugs will wear in so I wouldn't start cutting coils off till broken in. Then I'll buy a flat wire spring and cut till my bolt lock starts to work. Also running a hyd buffer

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On 2/3/2018 at 12:54 AM, TGibe said:

I had heard this and forgotten. I will put some more rounds through it and test again. Thx.

Rudy @ Macon Armory has done some testing with the Guard.  I've spoken to him about your problem and he'd like to see pictures of the rounds that had problems.  Send the pictures to:  rudyk@maconarmory.com  

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On 2/3/2018 at 12:54 AM, TGibe said:

I had heard this and forgotten. I will put some more rounds through it and test again. Thx.

Keep us updated after you run 500 rounds through it.  I have some interest in the Guard ... but I have no interest in having to "tune" a $1,000+ AR9.

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I dont know that break in is as much of the issue as the buffer weight. I am running a lighter buffer to get 130pf 147 gr and 124 gr ammo to run 100%. 

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8 minutes ago, bwikel said:

I dont know that break in is as much of the issue as the buffer weight. I am running a lighter buffer to get 130pf 147 gr and 124 gr ammo to run 100%. 

 

Thx. What buffer weight are you running? Did you also change out the spring?

Edited by TGibe

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1 hour ago, Tampa-XD45 said:

Keep us updated after you run 500 rounds through it.  I have some interest in the Guard ... but I have no interest in having to "tune" a $1,000+ AR9.

I think for standard ammo there isn’t any tuning required. Just us shooters looking to tweak for competition. 

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I don't have a complete guard I built with the bolt barrel combo. I have run buffers from 1 to 4 oz and a standard carbine spring. 

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13 minutes ago, bwikel said:

I don't have a complete guard I built with the bolt barrel combo. I have run buffers from 1 to 4 oz and a standard carbine spring. 

 

Thanks! That's exactly the info I was looking for.

 

The CMMG lower comes stock with a heavy buffer and heavy spring. I will try a standard carbine spring and 4oz buffer - I bet it will run fine like that with lower PF loads.

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I agree and think you are headed in the right direction. 

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On 2/12/2018 at 4:24 PM, BartCarter said:

My 9 mm Guard's buffer is 2.93 oz and the BCG is 11.78 oz.

I weighed my buffer and it is also just under 3 oz.

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Terrible day at the range today. Couldn't get the guard to feed properly. I increased my powder charge to see how it would perform with stock spring, but on full 31 round mags with TF +10's it would not feed worth a crap. Also tried my MBX mega with the same problems. Once the mags were downloaded to 30ish they seemed to feed OK. Both mags function full (I only load to minus 1 from max) flawlessly in my blowback PCC.

 

I've attached a picture of the malfunction (yeah, i removed the dust cover). The tip of the bullet is dented, which likely means its hitting the top of the chamber. The rim of the case is getting also mangled\chopped away by the bolt lugs.

 

Any ideas?

Could it be my OAL of 1.130-1.135"?

What is everyone else running for OAL?

 

Unfortunately I got a call that my daughter was heading to the ER and had to cut my range time short before I could even trouble shoot the problems. She's fine now, but it kept me from figuring out what happened.

IMG_8053.thumb.jpg.a466734154cc8930a991c866278b2bf6.jpg

 

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Is this running glock mags? My ammo runs 1.15, and it will rip them just fine when stuffed with 33. Standard spring and carbine buffer, the hydraulic blietz buffer works too. Try single round and see if it locks back, if not cut coils till it does.

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11 hours ago, cking said:

Is this running glock mags? My ammo runs 1.15, and it will rip them just fine when stuffed with 33. Standard spring and carbine buffer, the hydraulic blietz buffer works too. Try single round and see if it locks back, if not cut coils till it does.

 

Thx. Yes - real glock mags that run perfectly in my blowback PCC. Would rather fix the problem with ammo or the gun than modify working mags...

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13 hours ago, TGibe said:

Terrible day at the range today. Couldn't get the guard to feed properly...

 

 

My factory Guard ran perfectly from day one with no break-in.

 

The Guard that I built ran perfectly after I clearanced my non-Guard upper ejection port to the Guard factory clearances.

 

How does the Guard run on factory ammo?  If there is a problem there, the CMMG warranty is excellent.

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2 minutes ago, BartCarter said:

My factory Guard ran perfectly from day one with no break-in.

 

The Guard that I built ran perfectly after I clearanced my non-Guard upper ejection port to the Guard factory clearances.

 

How does the Guard run on factory ammo?  If there is a problem there, the CMMG warranty is excellent.

 

Thanks Bart. What OAL and bullet are you running?

 

33 rounds and below function fine, but I need more testing time to be sure. 37ish and above rounds fail consistently as shown above (could be resistance or bullet angle under spring pressure). Even releasing locked bolt on a full 40 round mag is inconsistent.

 

Will try loading some dummy rounds for bolt locked release testing at home. Also will take a box of factory 9mm to test in fully loaded mags.

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