So I'm working on a science fiction story where the main character is the Capitan of The Defiance, the first 99.99 C (as in percentage of the speed of light the gun achieves when firing) basically it's a Corvette ship traditionally used for fighter screening that has been built around the biggest mobile artillery cannon that humanity has built.
To make a very very very long story short, the ship has a standard event horizon drive for interstellar navigation, and then four more of them to fire the gun, which is like.... A lot of energy.
The main question I have is this: What would a bullet designed to be fired with as much energy humanity can put on a weapons platform? What would govern the shape of it, the mass, and the material? How would you imagine it to behave in flight?
I ask because I want a diverse range of opinions, and since this is based off of an HFY story that I really liked (with permission and credited) I figured I'd ask reddit to get your opinions on it