Base bleed

Base bleed is a system used on some artillery shells to increase their range, typically by about 30%.

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 low-pressure area left behind the shell due to its blunt base. This drag is difficult to remove, because the shell must be "nose heavy" in order to have proper ballistics, and cannot easily be shaped into a more aerodynamic form.