if you'd like to go for the pump action attachment, i think that, to be more accurately correct about the weapon IRL it should be like this:

if you use any kind of special ammo such as Slug Flechette and dragon's breath, it would let you use the semi automatic mode (this is basicaly how the weapon works IRL). but if the player uses normal ammo, or buckshot, it would force the player to use pump action.

the reason behind this is that, like the name SPAS says (Special Purpose Automatic Shotgun), is that only special kind of ammunition can generate the suficient amount of gas to make the gun ready to shoot again, and normal ammunition, or even, rubber bullets, can't generate that required amount, making the user only able to use pump action in order to feed the gun with another cartridge.

this is just something that i like to see in guns, the cool mechanics of the weapons, this should give the gun a little more special mechanic that would keep people wanting to use it more, because its something different, something new.