The Radial Delayed Blowback (RDB) system was created from the ground up by CMMG to eliminate problems with traditional blowback systems found in pistol caliber AR-15's. The biggest advantage of the RDB system is a gun that weighs less while simultaneously having less felt recoil. With a traditional blowback system, the only thing keeping the chamber closed is the weight of the bolt and action spring tension. As a result, the more powerful the cartridge is, the more weight is needed to keep the bolt closed long enough to safely operate. With the RDB system the bolt must be forced to rotate and unlock itself before it will cycle. This slows things down enough for chamber pressures to drop to safe levels ensuring reliable, and safe, operation without the need for heavy bolts or stiff action springs. Radial delayed blowback was an advancement from straight blowback. Compact action radial delayed blowback was an advancement to the original radial delayed blowback. This (fixed ejector on ...