This is an interesting question. A more generic question is whether there is some substance that is compressible (so as to store energy) but not so compressible that it does not return the energy to the ball. This is a question that is worth thinking hard about and worth doing some experimental measurements to study the effect. Such experiments are currently in the planning stage.

And the Bottom Line?

It is quite unlikely that corking the bat will produce any appreciable effect, either of a beneficial or a detrimental nature, on the distance of a long fly ball. It is likely to result in higher batting averages for contact-type hitters.

Update

In July 2003, the crack team of Professor Dan Russell of Kettering University, Professor Lloyd Smith of Washington State University, and I did a series of measurements on several wood bats provided by Rawlings, to whom we express our thanks and gratitude. The measurements utilized the bat testing facility at the Sports Science Laboratory at Washington State, of which Lloyd is the founder and director. The test consists of firing a baseball from a high-speed cannon at a speed of approximately 110 mph onto a bat that is clamped at the handle to a pivoting structure. The speed of the incoming and rebounding ball are measured, and kinematic equations are used to determine the ball-bat COR.

The primary bat we used was a 34” bat with an unmodified weight of 30.5 oz. The unmodified bat was impacted a total of 6 times. Then a cavity 1” in diameter and 10” deep was drilled into the barrel of the bat, reducing the weight to 27.6 oz. This “drilled” bat was impacted a total of 6 times. Then the cavity was filled with crushed-up pieces of cork (from wine that I had enjoyed the preceding two weeks!), raising the weight to 28.6 oz. This corked bat was impacted 12 times. Then the cork was removed and the drilled bat was impacted again 5 times. Unfortunately, the bat broke at the handle on the last impact. We had intended to fill the cavity with superball material, but that part of the experiment was cut short by breaking the bat. All impacts used the same baseball and all were at the same location, 5” from the barrel end of the bat. Various checks were done to assure that the properties of the ball did not change in the course of the measurements. A summary of our results is given in Figure 2. These data demonstrate that there is no measurable trampoline effect when a wood bat is drilled or corked.