The SKS has a conventional layout, with a wooden stock and rifle grip. It is a gas-operated rifle that has a spring-loaded bolt carrier and a gas piston operating rod that work to unlock and cycle the action via gas pressure exerting pressure against them. The bolt is locked to contain the pressure of ignition at the moment of firing by tilting downwards at its rear and being held by a lug milled into the receiver. At the moment of firing, the bolt carrier is pushed rearwards, which causes it to lift the bolt, unlocking it, and allowing it to be carried rearwards against a spring. This allows the fired case to be ejected and a new round from the magazine to be carried into the chamber. The SKS represents an intermediate step in the process towards the development of true assault rifles, being shorter and less powerful than the semi-automatic rifles that preceded it, such as the Soviet SVT-40, but being longer (10 cm or 4in) than AK-series rifles which replaced it. As a result, it has a slightly higher muzzle velocity than those arms that replaced it.