The SPAS 12 was designed to function primarily in semi-automatic, with the pump-action mode used to reliably fire low-pressure ammunition

The SPAS 12 was designed to function primarily in semi-automatic, with the pump-action mode used to reliably fire low-pressure ammunition such as tear gas rounds or less-lethal bean bags. Switching between firing modes is done by pressing a button under the foregrip, and sliding the foregrip slightly forwards or backwards until it clicks into position.
The SPAS-12 has a magazine cut-off feature that can prevent the loading of a new round from the internal magazine when the gun is cycled. This allows the operator to load a specialized round into the chamber without going through the entire magazine first. Another unique feature of the SPAS-12 was the hook seen on folding stock variants. This hook could be rotated in 90 degree increments so that it would fit under the user's forearm when the stock was extended. With the stock supported under the forearm the gun could theoretically be fired with one hand, allowing the user to fire around cover or use their support hand for other tasks.
Early SPAS-12 models featured a lever-type safety, but over time it would begin discharging the firearm when switched on or off. This was eventually recalled by Franchi and replaced by a push-button crossbolt safety.
The barrel of the SPAS 12 was externally threaded to accept a wide variety of attachments, from chokes to gas grenade launchers. One interesting and particularly rare attachment, called a "diverter", spread shot vertically or horizontally. All barrel attachments are considered rare, and demand premium pricing on the secondhand market.