A bat that has been flattened or altered to improve performance is an illegal bat. If such an illegal bat is detected before the first pitch, the batter shall be called out and the bat shall be removed from the game. If an illegal bat is detected after the first pitch, legal or illegal, the batter shall be declared out, and base runners shall not advance as a result of a batted ball. The bat shall be removed from the contest.
[OBR 6.03a4:If a batter uses or attempts to use a bat that, in the umpire’s judgment, has been altered or tampered with in such a way to improve the distance factor or cause an unusual reaction on the baseball, no advancement on the bases will be allowed (except advancements that are not caused by the use of an illegal bat), and any out or outs made during a play shall stand. In addition to being called out, the player shall be ejected from the game and may be subject to additional penalties as determined by his League President. A batter shall be deemed to have used or attempted to use an illegal bat if he brings such a bat into the batter’s box.]