Summarize the following:
It should be able to build and maintain pressure well, be made of high quality plastic, and hold a good amount of liquid. You'll need a metal bracket, either flat or an L bracket depends on the gun you buy. Attach the bracket to the gun, so that the flat metal comes out parallel with the ground, about 1" below the nozzle. Wrap the gun around the nozzle with tinfoil, so that only the open is exposed (and exposed as little as possible). This will help keep the gun from melting. Fill the water reservoir with lighter fluid. Attach a candle to the end of the bracket. You can use tacky, gum, or anything that isn't glue. Light the candle using your preferred method. Don't forget to pump it first.

Summary:
Get a high quality water gun. Get a metal bracket. Wrap the gun. Fill the gun. Attach a candle. Light the candle. Shoot the gun.