How much does barrel length affect velocity?

Muzzle velocity (ft/s) Range of barrel length to which data applies (inches) Approximate change in velocity for each 1 inch change in barrel length (ft/s)
Up to 2000 20-26 5
2001-2500 20-26 10
2501-3000 20-26 20
3001-3500 20-26 30
3501-4000 20-26 40