With turkey season on the horizon, studious turkey hunters will notice myriad new choices in turkey loads and begin thinking they should try one. Then, which shot to use? No. 4's? No. 6's? Which one patterns better in the gun? And which will penetrate better?
It's commonly held that if all else is equal (range and velocity) smaller shot will penetrate better than larger shot because it has less surface area to impede penetration.
To borrow straight from that all-knowing BullShooter's Bible, the NRA Fact Book:
The fallacy in [this idea] is obvious when it is carried to its logical conclusion, that a round cannonball would penetrate a much shorter distance than a small shot pellet.
The belief would be correctly founded if penetration by shot pellets took place as the result of an outside force applied t the pellets during penetration, pushing them through the target. This situation does not exist.
The only force carrying a projectile through its target arises from its own velocity and weight. Assuming like velocities, then the only factors making for different penetrations by non-deforming round shot will be weight and area. The weights of spheres of the same material will be to each as the cubes of their diameters.
Lets skip a bunch of mathematical formulas...
... Penetration goes up strictly in accordance with the diameter of the shot.
Conclusion for Turkey Hunters
It's proven that a No. 6 shot pellet will penetrate the head of a turkey at 60 yards, so unless you plan to shoot further than that—in which case I'd suggest searching for a new call rather than a new load—pick the load that patterns best in your shotgun, and then go hunting.