Originally Posted by MadMax
Blanks do not properly cycle a gas operated automatic firearm because there is no obstruction in the barrel to generate a high enough pressure to work a rifle action. Gryphon is correct in applying Newton's 3rd law of balenced forces. However, I am not sure if Gryph is asserting that the reaction force of the bullet directly actuates the recoil mechanisms. Applying force to the bullet has no effect on the recoil mechanism until the bullet passes the side tap. Before this stage, the bolt in an auto rifle is locked against the breech by the bolt carrier or some similar arrangement.
This is assuming that the weapon is gas operated. In the case of the MP5 for instance which uses a roller-locked delayed blowback, there is no gas system and the bullet accellerating in one direction provides the resistance necessary for the burning propellant to force the bolt the other way. The roller locks merely delay the recoil of the bolt long enough for chamber pressures to drop to a safe level. Other guns such as the Sten don't even have any locking system and rely solely on the sheer weight of the bolt to provide the delay necessary before extracting the spent cartridge case.