Here's a short clip from Nitro Company's web sight. They make some of the turkey loads around and know theri stuff. Hope it helps

The modern turkey hunter sacrifices pattern performance and velocity for maneuverability of a short 21" barrel. A gun with a 26" barrel will give you higher velocity and better patterns. For every inch up to 26", you gain 7 1/2 to 15 feet per second in velocity, depending upon the gun. A 26" barrel will give you 10 to 15 percent better patterns than a short barrel with the same choke constriction.