ok i thought so to, but, i tried again, this time i bought every mod there was not one of them would go in the slot. Maybe im doing something wrong. The slot is actually called a augment slot (the weapon is a assult cannon with 0/1 mod slot. But i still cant find a use for any mod you can buy except for my armor i can put mods in armor just not weapons. I just get message no available slot found. must be a bug.