hypervelocity ammo. 
if u dont know what it is, in short, hypervelocity is the speed that mostly planets, stars and asteroids reach. its so fast taht when they impact with other things, it causes the projectile and target to vaporize, and since it travels so fast, the vapor itself becomes dangerous. theres one star in milkyway that is called “hypervelocity star”. u can google it, theres some pretty cool images. i started a thread about it earlier, about US armys railgun which is a prototype that builds on hypervelocity. 
altho i dont think the scaled down US hypervelocity cannon looks as badass as the planetary projectiles. and since its sci-fi u look for you could probably do a hypervelocity star projectile. maybe the impact could look like this:
if that white thing in the left is the projectile after impact. and the blue is the sphere that is vaporized after the impact?