The Amnesia and Swapping Gun

It was a ray gun.

It had two buttons. One would swap people with...anything, really. He could swap people with other people, or with objects, and the world would not notice any difference.

The other button was an amnesia button. If aimed at a person, every single memory in their head would be erased and stored in the gun. John could then rewrite the person's life bit by bit, and even give their old memories to someone else.