Roller-delayed blowback was first used in Mauser's "Gerät 06H" prototype. Roller-delayed blowback operation differs from roller-locked recoil operation as seen in the MG 42 and gas operated roller locked, as seen in the "Gerät 03" and "Gerät 06". Unlike the MG 42, in roller-delayed blowback the barrel is fixed and does not recoil, and unlike the "Gerät 03" and "Gerät 06" and StG 44, roller-delayed blowback systems lack a gas piston. These omissions are conducive to relatively light construction by significantly reducing the number of parts required and the amount of machining required to produce a rifle. As the bolt head is driven rearward, rollers on the sides of the bolt are driven inward against a tapered bolt carrier extension. This forces the bolt carrier rearward at a much greater velocity and delays movement of the bolt head. The primary advantage of roller-delayed blowback is the simplicity of the design compared to gas or recoil operation.