Well, it sounds like a niche market type of thing at best.  A few folks are making launchers for golf balls, tennis balls, soda cans and even things the size of bowling balls.  Propulsion depends on blank cartridge adapters for the smaller stuff and black powder and other low grade explosive compounds for the larger stuff.  I've heard of some exotic compressed gas launchers but outside of the "Punkin' Chunkin'" competitors I've not seen any working models.  These folks carry some of the stuff mentioned:  http://unammo.com/

I guess if it were me I'd try to find a bread and butter product or two that has broader appeal and hang my hat on that until I got an actual workable business model for investors or a crowd funding appeal.  Assuming you want to work that hard and not just putter around building fun stuff.   

I was thinking about ordering one of those can launchers from X-Products, probably the .308 version.   Then the company CEO (and the crazy b____ he is married to) decided to start pushing for gun control.    It's a shame too, it really seems like a fun novelty product.

I can tell you from experience that the airsoft m203 runs on propane shells using their compressed volume to launch things. You can also use co2 and shoving other things down the barrel can be allot of fun;)

Just a thought on guys who want to fiddle with some sort of "launcher" system. Myself I 3d print out shells that fit over the propane shells that launch a long ways for fun;)

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So, I guess this is what you do when the government doesn't let you have real guns, eh?   I like the ambition and the work ethic but I'm not likely to embark on anything nearly so involved.  Thanks for sharing it with us though.