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 the fore-grip forwards or backwards until it clicks into position allowing the rotating sleeve to open or close the two gas ports.
The SPAS-12 has a magazine cut-off feature, which prevents loading a new round from the internal magazine when the gun is cycled. This lets the operator load a specialized round into the chamber without firing the whole magazine first. A unique feature of the SPAS-12 was the hook on folding-stock variants. This hook could be rotated in 90-degree increments, so it would fit under the user's forearm when the stock was extended. In theory, with the stock supported under the forearm, the gun can be fired with one hand; an example would be letting the user fire from a vehicle through the passenger-side window while driving.