Advanced Primer Ignition
Advanced Primer Ignition is used in open-bolt submachine guns in which the chamber depth is made a few thousands of an inch shorter than the cartridge case's length. This causes the forward moving bolt's fixed firing pin to ignite the primer a moment before the bolt strikes the chamber face. As the recoiling cartridge case begins pushing rearward, it meets the resistance of the still forward moving bolt. The bolt's motion is reversed by the recoiling cartridge case. Part of the energy of the two opposing forces is canceled resulting in both reduced felt recoil and makes the firing cycle seem smoother to the operator.
Advanced ignition also enhances controllability by reducing the submachine gun's muzzle climb. The heavy telescoping bolt's center of mass is forward of the submachine gun's center of gravity at the point of cartridge ignition. The telescoping bolt's inertial action pushes the submachine gun's muzzle forward and down, thereby reducing felt recoil and countering the recoiling cartridge's attempt to make the muzzle rise.
The inertia of an advanced ignition submachine gun's forward moving bolt must be overcome by the recoiling cartridge case. This allows the weight of the bolt to be reduced, decreasing the submachine gun's weight.
