With the release of the CMMG Guard’s radially delayed blowback, I couldn’t wait to get my hands on it as an alternative to the MP5.
With the AR family, technology has improved with new buffer designs as well as many different spring and buffer tube combinations which should continue to evolve as more people are using AR’s for competition, hunting, etc..
As seen here: https://www.ar15.com/forums/AR-15/Thoughts-on-the-CMMG-Guard-/15-713060/?page=10
AR15.com member, ‘Doctor_Evil’ had already started some of the legwork with some various buffers, buffer tubes and springs.
Based on his testing and I had done my own testing here.
Regarding the Tubb flat wire springs, what is interesting is that his 556 spring is advertised to work in a carbine OR rifle buffer tube.
Check out this link by David Tubb: https://www.youtube.com/watch?v=8fMVZbrnvu8&t=153s
If you don’t have the patience to watch the video, ‘Joshv06′ documented all the load sensor values:
A2 (old) – 7.6lb closed, 14.5lb open A2 (new) – 9.1lb closed, 16.7lb open
Tubb Flatwire – 10.5lb closed, 16.3lb open
Tubb .308 Flatwire – 13lb closed, 16.7lb open
SpringCo White ? (standard) – 8.3lb closed, 16.3lb open
SpringCo Red (extra) – 10.5lb closed, 18.4lb open
SpringCo Orange (increased) – 13.9lb closed, 24.8lb open
Use of the rifle length spring has always been praised for its smoothness.
Notice the new round spring is 16.7lb open, now notice that both Tubb Flatwire springs are in the 16lb range even the 308 spring. While also the closed positions are noticeably higher than ‘equivalent’ round springs, this means more force to help strip rounds off, get through crud, etc. VS a traditional round spring, if you try get that stronger closed bolt strength in a round spring, the strength of the spring is considerably higher in the open position while the Tubb Flatwire is topping out at the 16lb ‘sweet’ spot range. This is going to result in a smoother operation since it is a more consistent force on the BCG travel.
All those values were obtained by shoving those springs into a CARBINE length buffer tube in the video link above.
So logically, I would think as you go with a longer buffer tube like the A5 then Rifle, the numbers would get lower since the spring is under less compression.
I have found that using the A5 length tube, 556 Tubb Spring and 9mm Blitzkrieg/Kynshot buffer to work really well on not just the Guard but other configurations as well.
Here is some of that data. Note that RoF can vary quite a bit based on springs (buffer and hammer), magazines, ammo and how dirty the gun is. Even with testing on the same day, I don’t consider much of a difference if there is a change under around 40 RPM. I think any change that results in above that then it is considerable.
Beltfed FM9 - A5 length tube, 556 Tubb Spring and 9mm Blitzkrieg/Kynshot -
Rem 115Gr Green Box – 518 RPM
My 147Gr reloads 3.7Gr Titegroup (not suppressed) – 454 RPM
My 147Gr reloads 3.7Gr Titegroup (w/ MK9K) – 428 RPM
Shrike with A5, 308 Tubbs spring and 9mm Blitzkrieg/Kynshot.
In both tests below max gas setting was used
Short barrel M855 630 RPM
Short barrel Wolf Gold 542 RPM – only 9 rounds failed to feed 10th.
16” Barrel Low gas
M855 662 RPM only fired 9 rounds, failure to feed 10th round
Wolf Gold – 5 rounds and failure to feed 6th round
16” Barrel Med gas
M855 693 RPM
M193 661 RPM
Wolf Gold only 4 rounds and failure to feed 5th round
16” Barrel High gas
Wolf Gold 574 RPM on 9 rounds, 10th round failure to feed.
Did not bother to test M855/M193 since it was working on the Med gas settings
Wolf Gold is noticeably harder to insert into the links than domestically made 223/556 and needs more energy to be stripped from links. This buffer/spring combination shouldn’t be used with non-US made brass.
Also note that I see many people post about having to lube and stretch links. I have an early Shrike (delivered 2006). I have never resorted to any lubing or stretching of any links. I would assume if you did, Wolf Gold and other non-domestic 223/556 would work with this spring/buffer arrangement.
12.5 Mid (Baby Govnah gas block w/ port optimized for suppressed only..I think it is .063″) Suppressed Brevis Ultra II - A5 length tube, 556 Tubb Spring and 9mm Blitzkrieg/Kynshot, Custom Young Mfg Chrome M16 LW carrier, LC M855 – 669 RPM
The 12.5 Mid is my favorite as it is the shortest I feel comfortable going with a mid gas system. It is noticeably smoother than a 12.5″ carbine configuration. It is not a common configuration so below is some data on the more common 10.3″ MK18.
Many people incorrectly think that the A5 increases the amount of the distance the BCG can travel. The distance the carrier can travel is the same otherwise the carrier key would slam into the RE. Carriers like the new Surefire SOB have a modified key to allow increased travel which obviously requires a different (shorter) buffer to work in a carbine buffer tube.
That said, the main benefit to the A5 system is its use of a rifle length spring. This got me thinking about the 556 Tubb spring which is advertised to work in the rifle or carbine buffer tube. Why go to the A5 if just using the Tubb spring?
The A5H2 buffer is 5.3oz and a carbine H3 is the same weight. So using the H3 buffer and the 556 Tubb spring in a carbine buffer tube yielded 770 RPM…..22 RPM difference, as I mentioned earlier, I don’t consider anything under 40 RPM to be significant.
Based on those results, I think one can get the attributes of the ‘stock’ A5 setup by just changing to the 556 Tubb spring and using an H3 buffer.
However, I’m not dismissing the A5 either. Using the Tubb spring with the A5 is even better….(and using a Hydraulic + Tubb + A5 is even ‘mo’ better, more on that later).
Pictured below is the standard A5H2 buffer, A5 tube and just swapping the spring for the 556 Tubb was 702 RPM.
A5 length tube, 556 Tubb Spring and 9mm Blitzkrieg/Kynshot, LC M855 – 716 RPM….at only 14 RPM difference between the above, I would consider that to be the same in regards to RoF but I think this is noticeably smoother than using the A5H2 buffer.
Now switching to an optimized gas port of .052″