Just as Jay was explaining, you need to change the damage type in the config section of the Weap_<weapon> you wish to modify.
You need to have damage higher then 0
Torpidity multiplier works better when damage is 5+
You can make a weapon have different ammo types, what you need to do is create multiple Weap_<weapon> files for each type of ammo, as they will tell the gun how to handle the different damage types