During semi-automatic operation, the SPAS-15 is gas-operated. An actuator rod drives the bolt carrier when the weapon is cycled. When the weapon is fired, the gas from the cartridge travels through a gas port and impinges upon the actuator rod, which then pushes the bolt carrier to the rear, causing the bolt to rotate and unlock, allowing the bolt carrier group to cycle to the rear.
When switched to pump-action operation, the same components are driven by moving the slide to the rear. The pump-action mode of operation is required to reliably fire low-pressure (less lethal) ammunition, such as tear gas rounds or less-lethal bean bags; these rounds will not reliably cycle the weapon in semi-automatic operation.
Switching between firing modes is done by pressing a button above the slide, and moving the slide slightly forward or rearward.
The barrel is chrome lined and features screw-in choke tubes, and the SPAS-15 features a folding stock.
The SPAS-15 features a 2.75-inch 12 gauge chamber.