9mm CMMG Guard Full Auto Tuning

Goals for this platform are:

  1. Soft shooting – I still think the MP5 is softer shooting
  2. Flat shooting – Minimize ‘dot’ bounce so gun stays on target. some equate this as being soft shooting but I think they are different but related.  Using proper form, I find that my current Guard configuration is flatter shooting than either MP5 I’ve been comparing to.  However proper form/technique needs to be used to obtain these results.
    **Since writing this, I’ve taken the MP5K PDW and Guard out several times and everyone that has tried both have all said they find my Guard setup to be more controllable.  Though, most of these people don’t have much full auto experience.  I find this surprising since I do still think the MP5 is softer shooting.  May just be the higher RoF on the MP5K PDW that gives them a hard time.
  3. Slow enough cyclic rate to easily pull single round bursts.  Here is where the AR platform really shines over the MP5, MPX and many other platforms.  This was achieved by selecting from a large assortment of buffer tubes, buffers and springs on the market today.  These options overshadow every other platform in existence and this doesn’t appear to be slowing down.
  4. Good suppression platform.  Like the MP5, the CMMG Guard utilizes a delayed blowback mechanism which suppresses very well.  Better than any blowback 9mm Colt pattern AR.  As I side note I also have a post sample Sig MPX and IMHO the MP5 and CMMG Guard suppresses better.  Too much noise comes out of the piston with the MPX.


As of Nov 2018 pictured below is where I am at on the tuning of the CMMG Guard:

572 RPM, softer, flatter and slower than shooting a stock CMMG Guard

  1. A5 or equivalent buffer tube
  2. 556 Tubb Flatwire buffer spring
  3. 9mm Blitzkrieg/Kynshot hydraulic buffer
  4. Insertion of the static bolt weight pin.
  5. 18-8 SS washer placed after the bolt weight pin to prevent spring distortion
  6. Spring is then inserted (pictured below is a conical compression spring, others tested as well)
  7. Custom 2.2 oz Tungsten weight is inserted in the carrier.  Depending on the spring, I made sure it was either flush with the end of the carrier or slightly protruding so it isn’t beating on the buffer.


I have tested with a 1.8 and 2.2 oz Tungsten weight.
Conical compression spring used for this confined space: https://www.mcmaster.com/1692K919
Previous spring testing was deforming the spring from the Tungsten weight compressing the spring onto the bolt pin.  Thick 18-8 SS washer is now used to prevent spring deformation: https://www.mcmaster.com/90377A144

I had tested with the CMMG Tuning kit which comes with 3 weights: 1oz, 2oz and the 3.5 oz (same as the stock Colt weight).
In my experience adding a weight that is pinned to the carrier also allows that weight to pound into your shoulder and makes it more bouncy.  Ideally, you only want to affect the initial recoil impulse.  Think about an MP5, the rollers don’t do anything when the carrier is to the rear.
By using a spring loaded weight in the carrier, the weight can further delay the blowback but on the rearward stroke has less of an effect as a static weight that is pinned to the carrier since it is free to move independent of the carrier.  Additionally, by using the hydraulic buffer, the buffer may also be absorbing energy from the little Tungsten weight as well as from the BCG.

Using a spring loaded weight in the carrier is nothing new.  I can think of two carriers in production today that use this:
The Master of Arms Nyx BCG: https://www.masterofarms.com/index.php?main_page=product_info&products_id=69

and the Surefire SOB carrier: http://www.defensereview.com/surefire-optimized-bolt-carrier-long-stroke-obc-ls-drop-in-bcg-and-h7s-buffer-system-for-better-weapon-control-reliability-and-durability-best-tactical-ar-15m4m4a1-carbineshort-barreled-rifle/

In both cases these carriers are for 556 and not compatible with the CMMG Guard carrier.  From my observations, the carrier is machined differently to accommodate the different bolt design as well as different cam pin path.

Regardless, while the attributes of having a spring on the backside of the weight makes sense in that it will slow the cyclic rate and give the magazine more time to feed, I also think that it can increase the amount of force going into your shoulder on the rearward stroke since the weight is captive to the carrier.  I have considered trying a weight on the backside of the weight in the Guard and leaving the weight NOT captive to the carrier but unless a new hole is drilled into the carrier, there just isn’t much room to mess with this.

Pictured below is the distance behind the bolt pin.  .887″ is not much room to work with but better than nothing. DistanceBehindPin

I had some left over tungsten from another project and cut off a piece.  The groove in the middle is where a hole was before.

I went to a surplus hardware store and sifted through various springs.  I selected two different strengths that had the right diameter but needed to be cut down.  One was pretty weak while the other was considerably stronger.

A note about RoF testing, it can vary quite a bit depending on ammo, springs (buffer and hammer), magazines…and how dirty/clean the gun is.  In this kind of testing, I try to do a baseline RoF test the same day immediately before testing anything as the gun is the same in regards to how clean it is.  Below is my baseline config I have been using w/ no carrier weight added yet.
A5 or equivalent buffer tube
556 Tubb flat wire spring
9mm Blitzkrieg/Kynshot buffer

622 RPMNoWeightBaseline622RPM

Next was to try the 1.8 oz Tungsten weight with the ‘weak’ cut down spring which was 630 RPM.  At 630 RPM vs 622 RPM, that was really about the same…like it made no difference.  I was kinda disappointed as I could feel no difference either.  It was like the weight/spring did nothing.

I moved on to the stronger cut down spring and this resulted in 602 RPM and I could feel the difference in smoothness and the dot stayed on target much better than before.

It is hard to quantify how controllable something is.  So while the RoF information is good to know, it has no bearing on the control ability of the gun.  As an example the static 3.5oz Colt weight results in 525 RPM but it is really bouncy and hard to keep the dot on target.Guard-556Tubb-9mmKynshot-A5-3_5ozColtWeight

Initial tests were positive but I knew the surplus cut down spring were just a proof of concept and I would need to get something better.  I found a crappy washer in my tool box that happened to have a .625″ diameter and figured I would try that to see how it would hold up.  Picture below shows how the cheap washer is being bent over the bolt pin.  This was only 30 rounds of full auto with mixed burst fire.  Looking at this, you know that the weight is clearly moving with considerable force and further delaying the blowback.

The washers I’m using now mentioned at the beginning of this article are .060″ – .090″ in thickness and made from 18-8 SS.
I have had no more spring deformation issues with this washer.

McMaster carries 3 different strength multiwave compression spring that have .30″ length and .625″ diameter.  6, 12 and 20 lb.

The 6lb was like the cut down weak standard compression spring and acted like it wasn’t even there.
I can’t find a picture of the RoF results I did for that one right now.

The 12lb worked pretty decent in regards to smoothness.  RoF not bad either.  659 RPM.  I know that is faster than the stock 622 mentioned earlier but this was also done a few weeks earlier and the gun was probably more lubed at that time.

The 20lb wave spring at 612 RPM was getting results like the strong cut down conventional spring and was very nice.  This was done the same day as the 12lb test above.20lbWave

However, not leaving well enough alone, I felt that having to resort to this thick washer didn’t allow for much movement and the wave spring doesn’t have much deflection.  So I also ordered the conical compression spring mentioned earlier.

Conical compression spring with the 1.8 oz Tungsten weight 634 RPM and very nice.

A note about the concical compression spring.  See picture below of new unused conical spring.  Circled area shows how the last coil at the bottom is directly below the coil above it.  You need to bend that out of the way or cut it or it will get pinched off.conical-top

Picture below shows that the weight was coming forward so hard that it was able to pinch off the coil below it.

I had some more Tungsten laying around so had another piece cut to .681″ which is 2.2 oz.

That 2.2oz Tungsten weight, conical spring and SS washer is the current setup and I’ll be running it hard for a while to see how well it holds up.

I am also using the new Mean Arms Endomags.  I had tested using unmodified Colt mags using a modified Hahn mag block and modified mag catch as well as the CMMG Guard Glock lower.  I’ve always hated Colt mags and have had them fail on me too many times as well as ‘volcano’ing’ if slightly bounced.  I have also found that the Colt mags require more force to strip rounds off than the Glock or new Endomags.  More force required to strip rounds means a smaller operating window of reliability.  Again, I have specific goals in mind.  If cyclic rate is not a concern then going with a different buffer/spring arrangement would be fine w/ Colt mags but not for my goals.

So for me, it came down to the Glock or Endomags.  I really like double feed mags more than single feed for ease of loading.  So I am currently using the Endomags.  I really hope they come out with a 40 round version at some point.  The Endomags have been pretty reliable although I occasionally have an issue where a round is not fed, like it couldn’t keep up.  There is no jam, just no round in the chamber.  I have 8 Endomags for the Guard and just haven’t spent the time to figure out if it is specific mags or what.

As mentioned in the goals I want, the Sig MPX is not desireable to me due to its lack of tuneability and the suppression being superior on the MP5 and Guard so I would regularly fire the MP5 for comparison.  Below is what the Guard was up against.

MP5K PDW below that is running at 907RPM.

I would also compare to a full size MP5 running at 789 RPM:

Also tested against a Post Sample Sig MPXhttps://www.youtube.com/watch?v=XKVgczZKObI
In my opinion the MPX does not suppress as well as the MP5 or CMMG Guard.  The RoF is also high and like the MP5, both are NOT tunable for cyclic rate.  Sig MPX was eliminated from my comparison testing since there is nothing I can do about its suppression performance relative to the MP5 and Guard.

Historical testing info below

I was previously using the JP captured setups in several configurations (9mm straight blowback and 5.56).  I have the spring tuning kits, standard version in Gen1 and Gen2, Gen2 H2 version for full auto as well as the 9mm version (straight blowback).  Initially tried the H2 version and it was too heavy and was getting malfunctions.  Tried the standard weight and was getting light primer hits and cycled way too fast.   I decided to ditch the JP system since that system wouldn’t allow me to add weight to the carrier.

Did some preliminary testing on smoothness with the Tubbs flat wire comparing to the Sprinco Green and no-name AR rifle spring and think the Tubbs flat wire was the smoothest.  After that did all testing with the Tubb’s flat wire.

Testing below was with custom 5.1 oz tungsten carrier weight.
H2 RoF too fast and couldn’t get singles
H3 As seen on timer only got 7 rounds tried another time and got to around 8 rounds before malfunction again.  Was also bouncy
The Enidine Hydraulic was noticeably stiffer to compress than the 9mm Blitzkrieg/Kynshot
Blitzkrieg/Kynshot seemed to be the smoothest but did encounter some malfunctions which I think was due to just too much weight.
AAC tungsten was kind of bouncy and didnt’ lock the bolt back.
MGI was also bouncy and didn’t lock the bolt back.
The Griffin Armament AR SOB was surprisingly quite good and I’d say the best choice behind the hydraulics….Enidine and Blitzkrieg/Kynshot.
Did a little bit of testing with no weight and it was just too fast although quite smooth.
Finally switched to the 3.5 oz Colt Steel carrier weight.
Didn’t encounter anymore malfunctions with the Blitzkrieg/Kynshot, singles were quite easy to get.
Enidine was noticeably more bouncy than the Blitzkrieg/Kynshot.  Wonder how an carbine Kynshot would do compared to the Enidine since the Blitzkrieg/Kynshot is not as stiff.
** Update – ended up getting a Kynshot carbine buffer and the stiffness seems identical to the old Enidine.  Interesting that they both feel stiffer than the 9mm ‘Blitzkrieg’ version. **
Again, Griffin Armament AR SOB was pretty good and would probably use that if was not able to use the Blitzkrieg/Kynshot.
All pictured RoF results below were with a standard steel Colt 9mm bolt weight (3.5 oz) except for the AAC 7.4 oz buffer was done with 5.1 oz tungsten carrier weight.

Enidine Hydraulic buffer – 4.3 ozEnidine-A5-Tubbs 

H3 buffer 5.3 oz

Slowest ROF AAC 7.4oz buffer w/ 5.1 Tungsten carrier weight – bouncy and didn’t lock bolt back.


Griffin Armament AR-SOB buffer – 4.8 ozGriffinSOB-A5-Tubbs 

Blitzkrieg / Kynshot 9mm Hydraulic buffer – 5.9 oz.  3.5oz Colt weight was installed in the carrierKynshotBlitzkrieg-A5-Tubbs

A5 Buffer Length – HeavyBuffers.com

Blitzkrieg/Kynshot 9mm buffer (RB5007) – compressed

Blitzkrieg/Kynshot 9mm buffer (RB5007) – extended

Blitzkrieg/Kynshot 9mm buffer (RB5007) – weight

** Update, after more testing, I removed the weight from the carrier which has increased my RoF.  Same buffer system:
556 Tubb flat spring
9mm Blitzkrieg/Kynshot buffer
A5 length buffer tube

702 RPM

Same configuration as above using the Tubb 300BLK spring, I get 674 RPM but haven’t done a significant amount of testing with that spring.