8″ 40 SW CMMG Guard testing

I was hoping that the same buffer / spring combination I was using in the 9mm Guard (which also happens to work with just about every 556 upper as well) would work on the 40SW Guard but it turns out it was just unreliable.

Again, my 9mm Guard configuration is:

  • 556 Tubb flat spring (300 BLK Tubb flat spring also works well)
  • 9mm Kynshot Hydraulic Buffer (RB5007 – 5.9 oz)
  • A5 or equivalent (like Palmetto PA-10 buffer tube)

Every malfunction that occurred was not being able to push the round fully into battery.
I also tried the following configurations with the same malfunction:

  • 556 Tubb flat spring + Kynshot Hydraulic RB5005 (308 collapsible, 4.7 oz) + spacer (since using A5 tube)
  • 556 Tubb + Griffin SOB (4.9 oz) + spacer
  • 300 BLK Tubb + RB5005 (4.7 oz) + spacer
  • 300 BLK Tubb + Griffin SOB (4.9 oz) + spacer
  • 556 Tubb + H (3.7 oz) + spacer
  • 556 Tubb + H2 (4.5 oz)  + spacer
  • 556 Tubb + H3 (5.3 oz) + spacer

Finally tried 556 Tubb + MGI (6.8 oz) heavy buffer + spacer and that was finally a reliable configuration for me.

Also tried the stronger 308 Tubb spring w/ the RB5007 and Vltor A5H2 (5.33 oz) as well as all the above configurations and none were reliable.  Always the the same malfunction where it couldn’t force the round fully into battery.  Had to go back to the configuration above.

Forgot to bring the Vltor A5H3 (6.08 oz) with me.  Would like to see if that works with either spring.  Also  have the Vltor A5H4 (6.83 oz) which I need to try also and would think that would work as it matches the MGI weight but haven’t measured which one has more ‘moving’ mass.

Ammo used was primarily my reloads.  Some 180 Gr Berry’s plated bullets over 5.3 Gr WST.

Also tried some Precision frangible which I think was 135 Gr or something.

After getting it to be reliable, I then did  some cyclic rate testing.

600 RPM w/ my 180 Gr reloads and an old .40SW thread mounted SWR GSG40 suppressor.40SW-556Tubb-A5-MGI-SpacerDry-Can

Gun seemed dry and added some lube.  RoF went up from suppressed since it was lubed. 40SW-556Tubb-A5-MGI-SpacerLubed
It also ran 100% with old Scherer 31 round .40SW mags.