This was an old scifi book my grandma had. A modern-day man gets contacted by a prince in the far future, over radio, who explains that they can switch minds/bodies using radio waves, which can travel through time for some reason. They make the switch and he assumes the identity of the prince in the far future, aboard a cool starship, and has to fight political and military enemies. Some features of the story I remember:
- Bullets that explode after they enter a person.
- At one point, they land on a planet with people made of mud, and the protagonist gets captured and almost gets thrown into a big vat of mud.
- The climatic scene feature the unveiling of the secret weapon, which turns out to be a gun that
can destroy space itself. When they fire it, it destroys some space and everything crashes in to fill the destroyed space.
I think the title of the book had "Emperors" or something in it.