So, correct me if i'm wrong, but I assume this works: Make a barrel, with room for at least one cubic foot of sunmetal (more will make it more powerful), with a few inches of extra space and room for an equal amount in front. Put green (for radiation), red and orange (for explosion) kaleidoscopic filters in the loose space. Set up a circuit from behind that enters into the sunmetal. Load the cannon with sunmetal behind and a projectile in front (I'm partial to Phlogiston,) and blow the sunmetal. The filters should stop all negative effects, and the projectile should be shot at about 1000 miles/second (I think, could be much more.)
You'd need kaleidoscopic filters around the inside of the barrel too... Don't want all your force escaping out the sides, and most likely destroying everything around you.

That said, it looks solid. Just remember that this takes a whole lot of effort.