The answer is "It depends." It depends on what caliber you are shooting, what load, what powder etc. Usually the generic answer is 50-100 fps per inch +/- Some calibers/loads will reach optimal velocities in an 18-20 inch barrel, such as a 30-30 WCF. Some high capacity "Magnums" like a 7mm Ultra Mag or 30-378 Weatherby need 26" tubes or even do better with 28" I guess it has always come down to how much velocity do I need and how much barrel do I want to carry.
