The MDL bounding box is initialized with a default size of +/-32 units from the MDL center. To get a better fitting bounding box try calling c_updatehull. If you need really accurate collisions you will have to set the MDLs POLYGON flag to ON.

action hit_man
{
c_updatehull(my,0); // get accurate cube size
// or: my.polygon=on; to get polygon collision
my.event=hit_event;
my.enable_shoot=on;
}