About shooting from 75 meters above ground and not from ground level: Even though the projectile is shot on the same plane as the ammo, it still gets too much vertical thrust. The stone slab is to get rid of the vertical thrust. I can move the cannon down to ground level and test it so that the amount of vertical thrust is just the right amount, and I bet you I'll still get +100km. But honestly, I really don't want to work on this cannon anymore. I have a life. And most people don't care about these technicalities. It certainly doesn't help that the next update is literally within a week (at the time I released this video), which will break the cannon (see following paragraph).
About my claim that this is "the cannon to end all cannons": It's more than for the sake of having something dramatic to catch your attention. This cannon uses pistons at the highest speed possible, squeezing in as much TNT as possible. I highly doubt anyone will be able to develop a cannon of equal power under a week. The 1.3 update slows down pistons to 75% its original speed, so after the update, no one else will be able to squeeze in as much TNT as me. Unless they downgrade, but that's silly. Or unless someone comes up with a novel compression method. Someone up for this challenge?