A cannon that shoots electron-degenerate matter. This is stuff that you'd find inside white dwarfs, and it's hundreds of thousands of times as dense as (normal solid)tungsten. Armor penetration ability should be far beyond anything we are familiar with; Actually, I'm not sure what would happen to regular matter that touches it. But it probably can't survive outside of the enormous pressure that created it.

Behind the breech, there's a sphere of it, actively contained using electromagnets, or maybe artificial gravity. To fire, the containment around the breech is momentarily weakened, allowing a tiny piece of the sphere to detach because of the pressure. The containment is adjusted to re-center the sphere and stop the 'leak', and the piece is captured by the breech and accelerated further.

There's the question of how to keep the projectile from exploding too soon. If it can be compressed further, maybe it can be squeezed very hard and very quickly before leaving the barrel, giving its parts inward momentum, so during its flight it contracts before starting to expand. It should probably be accelerated to ludicrous speed, stretching the still-unexploded phase of its life over a longer range; Maybe even a longer time.