I think I've seen it before. Some guy made a large reservoir of BB's in a backpack I believe and called it a flamethrower.
Right here actually:
http://www.pipersprecisionproducts.com/FlameThrower.htm
From the video I remember it wasn't that great for accuracy but you could still hit a tree or bush at 30 maybe as high as 50 feet and get a some good bush penetration.