Crossbow like weapon. Shoots a fast moving rod of metal that does 200 damage and penetrates through people and walls. Slight gravity effect. Not meant to be in a "balanced" deathmatch, but maybe as an "Ultimate" weapon. Slow reload.
Preferably a simple design, with a set of rails and a boxy transformer as the main design features. I think it'd be quite cool to impale blockheads with an electromagnetically-propulsed iron spike.