I found this info at
http://www.chuckhawks.com/rifle_barrel.htmThe Lyman table shows the following approximate velocity changes:
For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.