The Sterling employs a degree of what is known as Advanced Primer Ignition, in that the cartridge is fired while the bolt is still moving forward, a fraction of a second before the round is fully chambered. The firing of the round thus not only sends the bullet flying down the barrel but simultaneously resists the forwards movement of the bolt. By this means it is possible to employ a lighter bolt than if the cartridge was fired after the bolt had already stopped, as in simple blowback, since the energy of the expanding gases would then only have to overcome the bolt's static inertia (plus spring resistance) to push it backwards again and cycle the weapon; whereas in this arrangement some of this energy is used up in counteracting the bolt's forwards momentum as well; and thus the bolt does not have to be so massive. The lighter bolt makes not only for a lighter gun, but a more controllable one since there is less mass moving to and fro within it as it fires.