The Trebuchet missile from Freespace 2.
Google up "trebuchet", and you'll start to understand. The acceleration is actually fairly gentle, but sustained. Trebuchets were the heavy artillery of medieval Europe.
I loved the scenes from the recent Joan of Arc flick. The trebuchet scenes were sooooooo fantastic. I really enjoyed some of those scenes. Pretty accurate too. I loved the way the guy in charge of the launch would chisel and get stones down to an exact weight in order to get them just right.
I said catapult, not trebuchet.