Made from hardened tool steel and parkerized. Service rifle legal! Has the same appearance and configuration as original; CMP 4th Edition #4-13-2-b. By adjusting the volume of gas in the cylinder you change the speed of the op-rod and the harmonics of the barrel. If you want to tame things down, it can be opened up just enough to function, or opened fully for single shot functionality. If you are working on your own hand loads, a quarter turn WILL affect group size. If you have a .308 Garand this is a necessity if you want to safely shoot heavier match bullets - 168 gr. Also a must have if you hunt with your CMP Garand but want to shooter heavier .30-06 loads. This is sold as new including instructions and the adjusting tool (allen wrench).