The Sterling submachine gun is constructed entirely of steel and plastic and has a shoulder stock, which folds underneath the weapon. There is an adjustable rear sight, which can be flipped between 100 and 200 yard settings. Although of conventional blowback design firing from an open bolt, there are some unusual features: for example, the bolt has helical grooves cut into the surface to remove dirt and fouling from the inside of the receiver to increase reliability. There are two concentric recoil springs which cycle the bolt, as opposed to the single spring arrangement used by many other SMG designs. This double-spring arrangement significantly reduces "bolt-bounce" when cartridges are chambered, resulting in better obturation, smoother recoil and increased accuracy. Additionally, the Sterling uses a much-improved (over the Sten) 34-round curved double-column feed box magazine, which is inserted into the left side of the receiver. The magazine follower, which pushes the cartridges into the feed port, is equipped with rollers to reduce friction. The bolt feeds ammunition alternately from the top and bottom of the magazine lips, and its fixed firing pin is designed so that it does not line up with the primer in the cartridge until the cartridge has entered the chamber.