with proper loads, 16-18" is all you really need... longer is dimishing returns.
Couger: there is a saying... if I have to explain it to you, you probably wouldn't understand it! I say the same thing to folks who say, "Heck, for $325 you could buy yerself a Savage Axis.... Why would you buy a cheap SINGLE SHOT?"
Joe S: if you are using a faster powder, and not pushing velocity too high, try your cast with some 45/45/10 tumble lube, and NO gas check... you may save yourself some $$$ and time, and have the same success