The spring rate is important, Matty, but it's only part of the mix.
What's most important is the amount of energy that the spring makes available to the piston (total energy minus energy in preload), the static force (the force on the piston in the 'at rest' position), and the force on the piston at the point of release. For any given spring, all of these will vary according to the piston stroke length and preload length.