Complex sentence: Open-bolt designs are generally undesirable for anything but belt-fed machine guns and pistol-cartridge submachine guns; the mass of the bolt moving forward causes the gun to lurch in reaction, which significantly reduces the accuracy of the gun.
Split into simpler sentences: Open-bolt designs are generally undesirable for anything but belt-fed machine guns and pistol-sized submachine guns. The reason is that the mass of the bolt, moving forward, causes the gun to lurch in reaction, which significantly reduces the accuracy of the gun.