Chaos Suggestion: Rail Gun.
Just think, you're wandering around, and you find this bad boy. You fire at a wall, a beam goes right through without killing any bricks, and kills who is on the other side.

For anti-abuse/spam measures:
-Make it have a charge-up time/cool-down time (ex: 5 seconds)
-Make the beam lag-friendly (not some huge lagging particle, just some simple, small green-ish beam)
-Little or no blast radius (rail guns usually are just a single projectile that hits and kills a single person)