I can't seem to find what defines in the box or out of the box in this situation. If he(batter) is leaving the box with one foot still in and one out and the bunt comes back and hits him. What is the call?
6.05 (f) a fair ball touches said batter before touching a fielder;
"The Right Call" Casebook -- Comment: When this happens it is almost always as a batter leaves the batter's box. Make sure the batter is out of the batters box when you call this, otherwise, call "foul ball".
The accepted interpretation is that the batter must be "completely" out of the box before you call him out. Completely means both feet on the ground outside the box.