Here are my suggestions, based on the picture and whatever the technology level in your game is.
Both of these work in the vacuum of space, and would need large barrels and energy storage, much like what you have
Both of these have already been mentioned by other posters before, but they are good.
Near Future Tech (With spaceship capability)
Mass Driver / Rail Gun / Gauss Gun: Shoots conducting / magnetized slugs at very high speeds
Far Future Tech
Singularity Cannon: Fires matter that has been compressed past the Schwarzschild limit, and accelerated to relativistic speeds.