It sounds like the short spring is the one that works for your setup. What is the lowest velocity that the gun will shoot at while using the short spring? This will give you a good indication of how it is working. For optimum performance, the operating velocity should be set at about 20fps above the lowest value where it starts cycling.
Originally Posted by Soonersdds
The higher pressure that causes the velocity to be over 300 fps could be a result of the barrel that you are using. A new spring pack doesn't change the pressure that it takes to move the bolt forward, so that won't help you.