Bat shaving has been proven to work by Penn State University. Gains of 62 feet of distance on a batted ball have been verified through their study. On average bat shaving gains are anywhere from 30-60ft add an end load to the bat and that number goes up even higher. Without a doubt bat shaving works and will cause a batted ball to travel much further than a normal stock bat.
Bat shaving is illegal in all baseball and softball associations. They all consider this to be altering a bat. The reason for this is once a bat is shaved the associations’ distance restrictions on bats is no longer within limits. Therefore, a shaved bat should only be used in certain settings such as batting practice (in a controlled setting) or a homerun derby contest.
With the right equipment, it is easy to tell if a bat has been shaved. Looking down the barrel by opening the end cap is the easiest way to tell. Another indicator could be a Barrel Compression tester. This will measure the stiffness of a composite barrel bat. If the barrel flexes below the allowable limits set by associations the bat could be shaved (but isn’t a tell all). One other way to tell is by simply hitting the bat and comparing it to another similar bat. If you find the bat hits much farther than the same bat your buddy has it is most likely a shaved bat. A properly shaved bat will increase the distance by 30-60ft; those distances should be obvious to the hitter.