Flamethrower: Probably legal. I have made custom flamethrowers (cannons and handheld, propane with a free flow valve, remote activation, or activation on a timer/programmable Arduino) and never had a problem. You need a permit from the city/county for a PUBLIC DISPLAY only. Bands get permits. I had people pull over along the road while I was launching 40 ft flames and 20 ft diameter fireballs, and then the cops showed up, they thought it was fine, not a public display because I wasn't advertising, just testing stuff.
Shotgun loaded with tacks? Makes no sense. Load the shotgun with shotgun shells.
Remote controlled tank from 4 wheeler: would have to be a big 4 wheeler to carry any sort of armor whatsoever. You would need to cover the propane tanks. It would be simple to use DC linear actuators to directly move the handlebar, but steering response would be slow. Another for the throttle. I'd leave it in first gear lol.
Much MUCH more difficult to design a tank from scratch. Maybe you could back-half a snowmobile, or replace the engine with a big DC motor and battery. Put some wheels in place of the skis...
Sounds like a lot of fun, right up until the barn is on fire and your car is blasted full of holes. Still, maybe worth it.