here's my 2 cents on non-linear springs.
let's look at the dynamics of the spring system in general. When any spring is compressed in free space, it will emit an equal amount of force on both ends.
now, when it comes to non-linear springs, it does matter which side goes front/back. What makes the difference is in the mass of the spring. One side is heavier than the other when balanced from the center (closely spaced side = heavy; widely spaced side = light).
If we install the heavy side forward, this will add mass and inertia to the moving piston which in turn causes the force of the spring to be reduced. Reduced force onto the piston means lower fps. Vice-Versa, lighter side of spring forward means less mass and inertia to piston causing more force from spring. More force from spring means higher fps, yay! Refer to my diagram attached.
This could explain the results from "jugglez" and "Stalker" from reversing these springs.