RifleShooter.com ran an article where they tested the effect barrel length had on velocity in a rifle chambered for the .300 Win Mag.  The author started with a 24.25″ barrel and cut off 1″ at a time.  At each length, groups were shot for testing both accuracy and velocity.

Velocity changes varied at each length, but a little more than 300 fps was lost going from the starting length down to 16.25″.  As one might expect, muzzle blast substantially increased as the barrel was cut down.

Check out the full article here.

Thanks to Bill for the tip.