Gas operated firearms have received wide acceptance in the marketplace. One of the main advantages of this type of firearm is that the shooter experiences less "kick" due to recoil than is the case for other types of firearm systems. This reduction in "kick" is partially due to the fact that the gas operated firearm utilizes a portion of the expanding gas released upon firing to reciprocally move the bolt assembly within the receiver, which movement automatically extracts and ejects the fired shell, recocks the trigger and delivers a new shell into the breach for reloading. Reduction in recoil or "kick" is also realized by venting excess gas, though initially routed to achieve the reciprocal movement, in a direction which minimizes its contribution to the recoil.
Almost universally the gas operated firearm relies upon a cylinder-piston arrangement to harness the energy of the expanding gas to accomplish the movement of the bolt assembly. In most instances the pressurized gas is routed to a cylinder which contains a piston which acts against or with a sleeve which is slidably mounted on the magazine portion of the firearm. The sleeve is often connected to the bolt assembly by at least one elongated slide arm. The slide arm is of substantial length since it is connected to the bolt assembly and must follow the bolt assembly throughout its entire movement. This substantial length is troublesome as the slide arms tend to bow when force is applied to them. To accommodate this bowing it is necessary to have relatively large apertures in the receiver through which the slides pass to make connection with the bolt assembly. The use of such large apertures is not desirable as the pressurized gas has a tendency to blow back into the receiver through these apertures. The result of this undesirable blow back is a deposit of carbon, powder fragments, etc. on the trigger group, reloading assembly, etc. which are located in the receiver.
One system which has been devised to allow the utilization of smaller apertures in the receiver through which the slide arm can operate is the one disclosed in U.S. Pat. No. 3,568,564 and U.S. Pat. No. 3,657,960. The system disclosed in these patents utilizes a relatively short push rod which is not connected to the bolt assembly. The push rod is positioned to contact an inertial piece and the bolt assembly during the work stroke. Since the push rod is not connected to the bolt assembly it can be designed relatively short thereby avoiding the bowing experienced by the longer slide arms during the work stroke. As there is little or no distortion of the push rod the apertures in the receiver through which it passes can be made for a fairly snug fit thereby reducing the amount of gas blown back into the receiver. Despite this advantage, this system has a significant drawback in that it relies upon the push rod providing the principal momentum to the bolt assembly to achieve its reciprocal movement within the receiver. In fact the thrust of the push rod against the bolt assembly and inertial piece must be of such magnitude that the sleeve, on to which the push rod is connected, is allowed to slide rearward until it impacts the front of the receiver. As can be appreciated, this impacting by the sleeve on the front of the receiver contributes to wear and tear, not to mention its contribution to "kick" felt by the shooter. Also the necessity of utilizing an inertial piece along with the bolt assembly contributes to complexity in manufacture and thus cost.