Man travels to future by switching minds over radio waves?

Question

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:

1. Bullets that explode after they enter a person.
2. 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.
3. 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.

Answer 1

It's The Star Kings by Edmond Hamilton. From Goodreads:

Flung across space and time by the sorcery of super-science, John Gordon exchanges bodies with Zarth Arn, Prince of the Mid-Galactic Empire 2000 centuries in the future! Suddenly John is thrust into a last-ditch battle between the democratic Empire World and the tyranny of the Black Cloud regime. Only one weapon—the terrifying Disruptor— can win the struggle for the Empire Forces. But it is so powerful that unless John uses it correctly it could destroy not only the enemy but the cosmos.