When filming an effect where a drunk shoots a roman candle into a tree and catches the tree afire, we had to hit the target spot (where the propane flame rig was set) exactly, in as few takes as possible. I didn't want to spend four hours shooting a scene that should take fifteen minutes, so I "wasted" the four hours in the machine shop, instead, and got a tool I could keep for other such purposes.
This precision comet gun allowed us to hit a 12" diameter target area from
30 feet away with perfect repeatability. We hit it on all five takes, and the director was able to select the take on the basis of how well he liked the fire that resulted, rather than on how accurately the comet hit.The gun is bore-sighted, and breech loaded with a small disc comet (22mm dia. x 8mm thick) with a lift bag attached. The "coin" comets seem to travel as accurately as "square" ones (go fig!), and, of course, they burn out quickly.
The turn-lock breech plug is fitted with a nichrome wire igniter that's good for about 100 shots before the element must be replaced (field replacable). It's sealed at the breech with two stepped o-rings. Firing is by 12v DC or AC supply, as required. It fires roughly 1/3 second after pressing the switch.
The breech end swivel block is made from 6061 aluminum, as is the base. The barrel is turned from #316 alloy SS for corrosion resistance. It's threaded
40tpi to allow a "short barrel" version that will diminish the maximum distance the comets will travel. On one angle shot, we had to keep the comet within 40' of the launch position. On the other shots, it could safely travel further, so the longer barrel afforded better accuracy.The whole thing mounts on a standard 1/4"-20 tripod. We shot it from a
5-in-12 rooftop, so the footing was a bit chancy!