As I recall... there were some fairly impressive arrow-launcher devices in medieval history that would fire some larger than usual bolts. I'm picturing an oversize crossbow on a carriage mount pulled by a pair of horses.
Seems perfectly suitable for the back of a pickup truck.
Hmm... one of those, a pickup truck, a six pack, and a bunch of rioting anarchist wannabe hippies... now we're talking quality entertainment. :-)