THROWN BALL STRIKES HELMET OR BAT

Rule 5.09(a)(8):

If a thrown ball strikes a helmet or bat accidentally (no intent on the part of the runner to interfere) in fair or foul territory, the ball remains in play the same as if it had not hit the helmet or bat.

If, in the umpire’s judgment, there is intent on the part of a runner to interfere with a thrown ball by dropping his helmet or bat or by throwing either at the ball, then the runner is out, the ball dead, and runner(s) return to the last base legally touched.