Projectiles and chambering are most of what I already have the capability to do well. I'll be getting a hold of my old paintball gun in a week or so, and soon after I'll ask around and see what it would take to get a barrel or three machined. Test a few different sized and weighted rounds, combined with different length/twist barrels, to find the most accurate, safest combination. After that, find an engineer and some investors, really.
My main design flaw is that it's very tedious to make a valve between the gas expansion chamber and the gas source. The only options there are: Use a piston system (which I want to avoid), or use an electronic system (which I also want to avoid) or have a very mechanically complex linkage between most triggers and the gas system (again, bad). I could make it slightly easier on myself to work with, say, an M4/M16 receiver, or any other carbine out there, instead of trying to cram it all in a chinese type 88, but the lack of cool points is putting me off.
Plus, my tech skills are highly lacking. I could make rounds with little difficulty, or make polymer/plastic shells for the outside of a gun, but I couldn't machine a polygonal barrel, or a bolt, or a linkage, or any of the pieces that hold them together and make them work.