Most of the drag on an artillery shell comes from the nose of the shell, as it pushes the air out of its way at supersonic speeds. Shaping the shell properly can reduce this greatly. However, another powerful source of drag is the vacuum left behind the shell due to its blunt base. This drag is difficult to remove, because the shell needs to be "nose heavy" in order to have proper ballistics, and it cannot easily be shaped into a more aerodynamic form.