TESTED: CMMG Radial Delayed Blowback

A lot of people don’t realize how challenging it is to develop an AR pistol or SBR that will feed and function reliably. In this story, we’ll look at CMMG’s Radial Delayed Blowback technology, learn why it was developed, and how it has a drastic taming effect on recoil.


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About Radial Delayed Blowback

Radial Delayed Blowback is a proprietary AR-platform technology developed by CMMG for PCC and related platforms. From the CMMG website:

A prime example of our Aim To Be Different mantra is our patented Radial Delayed Blowback system. The Radial Delayed Blowback system paved the way for larger caliber PCCs in an AR platform. Radial Delayed Blowback allows reliable operation without a heavy bolt, buffer or spring. The mechanical delay allows pressure to fall in the chamber before it unlocks.The system means less recoil, increased accuracy, and better velocity when using a suppressor. We started with .45 ACP and additionally, we offer 10mm, .40 S&W, 5.7x28mm and 9mm in our BANSHEE line.

This picture from the patent is a great visual showing how the chamfered lugs on the bolt are arranged:

As a part of the blowback action, these chamfered lugs unlock (as a part of rearward movement) and compress the internal spring which is seen in the above image. This gives the gas system more time to work and to lower pressures. The smoother action under recoil helps to prevent rounds of ammunition from “bouncing” out of the magazine during the cycling of the action.

CMMG Banshee 9mm Product Line Progression

The original version of the CMMG 9mm Banshee was inspired by the Colt SMG 9mm adaptation of the M16:

MK9 (SMG Magwell, Original Release)

This first version of the 9mm Banshee improved on the original Colt SMG platform with the following enhancements:

  • Flat top upper and freefloated picatinny handguard
  • Solid magwell (as opposed to pin-in blocks on SMG)
  • Enhanced parts collection

MkGs (Glock Magwell)

The MkGs has two key changes/improvements compared with the original MK9:

  1. Glock 9mm Magwell and mag catch
  2. Radial delayed blowback system integration

*Note that CMMG has incorporated Radial Delayed Blowback into the current MK9!

I personally prefer the Glock magazine configuration- I find these magazines much easier to load, and I have a bunch of them laying around from my Glocks! But what about the difference in recoil?

Comparison of Moving Mass

In addition to the mechanical/operational differences between straight blowback (MK9 Gen 1) and Radial Delayed Blowback (MkGs), there’s the fundamental issue of moving mass.

Here’s a comparison of the mass for key moving parts for each platform: (all weights in ounces)

So for total moving mass (minus the buffer spring which is the same) we have 42% MORE moving mass for the MK9 with straight blowback. That’s a big difference! How will this translate to differences in recoil? We’ll look at that next!

Introducing the Ultimate Reloader Recoil Rig

A long time in the making, I’m ready to unveil the Ultimate Reloader Recoil Rig with this story! Based on a design published by Cal Zant from the Precision Rifle Blog.

Here are some specs and details for this rig:

Unlike recoil sleds or pendulum setups, we can use this test rig to see in amazing detail how the recoil impulse unfolds in time (not just a “net effect”). Let’s see what that can look like!

Above: Firing the MkGs Banshee in the Ultimate Reloader Recoil Rig

Results from Testing: MK9 Gen 1 -vs- MkGs

Each of the Banshee AR pistols were used to fire a single shot mounted in the recoil rig. Each test produced 70,000 – 80,000 data points. So the first step in the analysis is to find the “moment in time” of interest. About 0.080 seconds total:

The data files were analyzed and synchronized producing the following graph:

Above you can see the differences between the straight blowback recoil sequence (orange line) and the Radial Delayed Blowback sequence (blue line). Here’s a summary:

  1. Initial impulse (RDB only) showing the lugs unlocking)
  2. Second peak: buffer and bolt bottoming out in buffer tube
  3. Bolt moving forward, resonance from #2

Clearly, the Radial Delayed Blowback has a more “spread out” and “gentle” recoil signature. I believe this is mostly due to the reduced moving mass, but is also due to the extended sequence of events: including the lugs unlocking.

When you shoot the rifles side by side, you can clearly feel the difference as well. It’s nice when the “science” and “experience” can agree like that.

Above: From the video, the moment when the RDB-equipped Banshee buffer and bolt bottom out in the buffer tube.

What’s Next

There are a LOT of tests we are looking at with our new recoil rig including:

  • Muzzle brake comparisons
  • Suppressor recoil impulse testing
  • Effect of bullet weight and powder charge on recoil
  • Effect of rifle mass on recoil
  • Effect of buttpad on recoil

If you have ideas for us, please leave a comment!

Get Your Own Banshee

If you are interested in your own Banshee, the best place to go is the CMMG Website:

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