Napalm cannon
Combine Electric Cross Bow , Machine design for bevel, spur and worm gears, Optical encoders, PID control, ICub robotics, Slam, Lidar, Quad frames, ESC speed control into a MeshNetworking ,Gprs and wifi streaming controlled tracked robotics platform.
Projectile is a napalm canister, a mixture of petrol(gasoline) and styrofoam. Airburst the canister with a small attiny AtmelMicro and kat.cr pyrotechnics over target.
Under South African law the joules energy may not exceed a certain amount on any type of CO2 based system. You cannot legally make a co2 cannon that will punch a titanium ball through an engine block. And come to think about it, one probably cannot legally make a sealed canister of napalm with a detonating device. If such a device is used , make certain that some sort of automated destruction is possible of the device. But there isn't any law against the energy content of a rope,wire based cross bow or catapult or is there?
An Electric Cross Bow is preferred as probably thousands of repetitive shots are possible. A co2 system though will have a higher rate of fire.