Paintballs, even FS rounds, are still very different than bullets. From what I’ve read, the FS does not start rotating until after it leaves the barrel. I would say stick the general 12-16 inches, but feel free to do some research. Let me google that for you
Most debate among FS rounds is the optimal bore. An underbore can damage the fins, and from what I’ve read most people say .689-.691 seems to be the best. FS rounds are cool, but those things be damn expensive doh.