PDI's Vacuum System could achieve the longest shots.The point is below.
When you pull the bolt handle,in the piston head,the rubber ball is free.
Next, you release the sear,the piston goes ahead,then the port is closed by the rubber ball.
At the last,attacking the cylinderhead, the ball is dropped and the Air comes in & clear the negative pressure.
That's why ,BB goes straight & you could achieve the longest shot !