In 2015, the CMMG company decided to develop a pistol-caliber AR carbine firing .45 ACP. Such things were becoming common in 9mm, but .45 was a more open market opportunity. They spent substantial time trying to perfect a simple blowback system, only to find that the recoil impulse was causing occasional feed failures that just couldn’t be rectified. They next tried using the regular AR “direct impingement” operating mechanism, but that wouldn’t provide enough energy to run the system reliably – and was very direct to boot. Just when the project looked hopeless, they took inspiration form the WWII Reising submachine gun – a delayed blowback .45ACP system.
Thinking about delayed blowback options, they hit upon the idea of using the AR cam pin as an accelerator lever. By cutting the back of the AR bolt lugs at 45 degrees, they created a system where the bolt would try to rotate and unlock under direct pressure from firing. This rotating acted through the cam pin to accelerate the mass of the bolt carrier rearward before the bolt head could start to extract the cartridge case. They named the system radial delayed blowback, and proceeded to spend 2016 perfecting the system, ultimately releasing it in April 2017 to the market. Following that, they expanded their product line from .45 ACP to include 9×19, 5.7×28, .40 S&W, and 10mm Automatic.
There are very few new ideas in firearms operating system these days, and the radial delayed blowback is a clever and elegant one, using an existing set of parts in a novel way.