Hey SeaMarc, It seems an interesting concept, but if your mate wants to really make money, I would think it would be a very niche market he needs to sell into. I think traditional band guns are simple - the roller gun will always be more complicated with more moving parts, and lot harder track through the water as less sleek. Inventors rarely access public opinion (so you are doing the right thing) and think what they have done is always the best - they have after all spent years on their project, however its the market (and the right marketing influences) that decides if its going to become a commercial reality. I think 93% of patents are never commercialized, so there are a lot of inventors out there that couldn't bring their product to the market or it was just not marketing (but interesting/fascinating), but the patent lawyers are very happy. I'm actually a pneumatic fan so maybe a shouldn't comment, good luck buddy!