Vacuum cannons use the 14.7 pound of force present in the ambient atmosphere to propel an object through a vacuum at incredible velocities.

This propulsion is possible because the absence of air in the cannon removes all of the air in front of the ball, then 14.7 pounds of force from normal atmospheric pressure rush in behind the ball launching it forward to fill the space back up.

Because air was removed, the ball has no air resistance to fight. The ball is also very light. This combination allows the ball to accelerate
nearly as fast as air can rush into the cannon.