Single solenoid systems are required to choose between closed bolt/fixed volume or open bolt/adjustable volume operation. Dual solenoid systems like the F2 and Fusion Engine allow for the nozzle and poppet valve to be adjusted independently of each other so you are able to achieve the best possible air efficiency while getting the performance advantages of a closed bolt system.Being able to fine tune the air volume which the system produces makes it possible to optimize air efficiency for a particular barrel length so it will only release the precise volume of air needed to fill that barrel. Fixed volume systems release a set volume of air so their air efficiency suffers in comparison.