I’ve found that I couldn’t get the same level of reliability using my preferred buffer tube, buffer (Kynshot RB5007) and spring (556 Tubb flat spring) setup I use for the 9mm and .45 CMMG RDB builds.  What would happen is failure to fully chamber the round.  

So, I’m currently running:

Vltor A5 tube

MGI heavy buffer (carbine length)

308 Tubb flat spring

New Frontier Armory spacer since I’m using the carbine length buffer in the A5 which allows the buffer spring to use the extra length but prevents the carrier from going back to far.  

I may mess around with A5 buffers later which is why I’m leaving the A5 tube installed at this time.

This yields a nice cyclic rate of 602 RPM.  It has been pretty reliable but the ejector springs still fail around 1K rounds like it does in 9mm.

Suppressor is an old SWR GSG40 with custom Curtis Tactical ‘Bixler’ 3 lug mount.  Almost 40 RPM increase in cyclic rate at 639 RPM.  Reliability suffers some when suppressed, I occasionally get failures to fully chamber the round.


Still having some reliability issues with this configuration .  It requires a good amount of force to fully chamber the round while at the same time if there is too much mass for it to overcome, it will have malfunctions on the rearward stroke.   

I am currently using a custom tungsten weight (5.1 oz) in the carrier. 

Removed the MGI buffer and replaced with a rebuilt Griffin Armament SOB buffer.  Same 308 Tubb flat spring, A5 tube and spacer.  

Cyclic rate went up about 30 RPM more with this lighter buffer when unsuppressed.

Suppressed, rate is similar to what I see in many of the 9mm RDB configurations with an approximate 60 RPM increase.  However, it is much more reliable with these components than anything else I have tested so far.  Even at this cyclic rate, I can still pull single round bursts at will.

9/6/20  Configured above ran several mags flawlessly yesterday then went to SMG match today and it started giving me malfunctions to fully chamber rounds again sometimes.  I went back the MGI buffer and unsuppressed and ran 1 stage clean.  Later starting to do the same thing again.  Did have 1 failure to eject so ejector spring may be starting to go again.  Need to measure how strong it is but I’d say I’m not even at 500 rounds yet.  I’m going to go back to the spring loaded tungsten weight in the carrier and try that.  If that with the GA SOB buffer.  I may try replacing one of both the of the GA SOB weights with tungsten as well.

10/10/20 – The .40SW RDB has been very temperamental for me.  Using all tungsten weights in the GA SOB didn’t help.  Ran about 500 rounds today with various configurations and have come to the following conclusions. 

  1. The custom 5.1 oz tungsten carrier weight has greatly reduced the issue of not fully chambering rounds.  I didn’t want to have to resort to adding mass to the carrier but it seems that this has worked the best for curing that issue.
  2. Other common malfunction is failure to feed where the round feeds at a steep angle and jams.  I am thinking this is being caused by too much BCG velocity.  I’ve gone away from the 308 Tubb flat spring to the 556 Tubb flat spring and that seemed to greatly reduce that malfunction from occurring. Note that I was using the 308 spring for additional force to chamber the round but the 5.1 tungsten weight in the carrier fixes that.
  3. As seen in the cyclic rate tests above, the .40SW can adds around 40-60 RPM increase in the cyclic rate…which increases the BCG velocity and causes the FTF malfunction.  Switching to a .45 suppressor which has a larger bore aperature and less back pressure has greatly helped.  I ran several mags using the same configuration I use for the .45 CMMG and many of my 556 builds has worked out well…556 Tubb flat spring, A5 tube, RB5007.
  4. The same buffer/spring configuration was consistently having malfunctions on the last round when un-suppressed. Swapping the RB5007 for the Vltor A5H4 seems to have corrected this issue.  However that same buffer doesn’t work as reliably as the RB5007 when suppressed.  So I need to use different buffers when suppressed and un-suppressed.

Pictured below is the suppressed configuration that currently appears to be reliable.  Suppressor is re-core by Curtis Tactical.  I plan on writing some more about this suppressor. It has excellent serviceability and performance.

Pictured below is the un-suppressed configuration that currently appears to be reliable.