The SPAS-12 was designed to function primarily in semi-automatic mode, with the pump-action mode used to reliably fire low-pressure ammunition such as tear gas rounds or less-lethal bean bags. The firing mode is switched by depressing the button under the fore-grip and also sliding it forwards or backwards until it clicks into position allowing the rotating sleeve to open or close the two gas ports. The semi-automatic mode functions with a short-stroke gas piston system. When a shell is fired, the pressure makes the bolt move rearward but the bolt will stop before opening as it deploys a single top locking lug. The gas vents into the barrel and the gas port is located towards the end of the barrel. From the gas port, the gas vents downwards into the gas block where it pushes the piston rearward. Because of the presence of the magazine tube and to avoid the use of a top-mounted gas system, the gas block and the piston are annular, along with the piston spring they enclose the magazine tube. Once operated, the piston drives 2 symmetrical thin sprung operating rods located in the corners formed by the barrel and the magazine tube. As the rods push the bolt carrier rearward, it disengages the locking lug and drives the bolt rearward, allowing the system to cycle.