I don't see the point in making the rf20 a single barreled gun. The purpose of that gun is for assault from close range using a barrage of darts. Instead of single barreling it, just use a super soaker.
I usually take a half inch drill bit and drill my cpvc out. It takes a hell of a long time and it is really hard to get it smooth but if you do it alot you get good at it. Also, i try to make cigar darts for larger diameter brass barrels. They're just darts with electrical tape wrapped around the end. This gives it a better seal with the barrel. You have to wrap it more than a few times though.
that depends on if you count modded water guns. one of our clan members has a modified water gun that shoots 500+ feet when tilted. When straight it shoots about 200-300 feet. It's insane. He shot my brother with it in a battle and it left a welt that was open and bleeding. INSANE!