You start with Computer/1 on the gun. You turn it into a specialised computer, which basically makes it Computer/3 but you can't change the programs built into it.
You then give it Intellect/1 and Expert/2. It doesn't need Intelligent Interface because the user isn't using the Expert program - the Intellect program is. The gun now automatically adjusts its targeting system, recoil etc to maximise the user's chance to hit.