Depending on the ball's composition, I can very well expect some of them to do a commendable job of acting as a mirror for incoming rounds. And if you should get something powerful to penetrate deeply, I'd expect the ball to shatter with considerable force, again, depending on the material it's made of. Hard rubber, old style balls should be the more "reflective" ones, and the newer reactive resin/urethane balls should be the ones that blow up.
Idea coming:
Know anyone who shoots black powder cannons? Find his bore diameter, exactly.
Have a machinist turn a bowling ball down to make a cylindrical projectile of the right size for the cannon, and turn the back flat or slightly concave. Put a nice ogive on it for ballistic reasons. Have your cannon-shooting buddy load it up and fire it.
Tape the action, and post it here.
CJ