The stiffness of a spring is a measure (called the spring ‘rate’) of how much force is needed to compress it per unit of distance, such as pounds force per inch, and so it is equally a measure of the force the spring can exert on the piston as it expands during the compression stroke.
The stiffness of a spring varies by its wire diameter raised to the power of four, so wire diameter is the dominant factor in determining stiffness, and the wire diameter of a spring can on its own tell you quite a lot about what the spring will probably do to the shot cycle of your rifle.
2.9mm diameter wire will produce a spring at the soft end of the range, which will need a fair amount of preload, to give a gentle recoil and reduced surge. For a given muzzle energy, the soft spring should generate lower peak cylinder pressure and temperature.
3mm to 3.1mm diameter wire is probably the nearest to a ‘standard’ airgun spring likely to be used by manufacturers for UK muzzle energy levels. It will need less preload than 2.9mm wire, will give a slightly quicker feel to the shot cycle, but with more surge travel.
3.2mm and above diameter wire will produce a stiff spring which, at UK muzzle energy levels, will need minimal preload. The shot cycle will feel decidedly ‘quick’, and there will be greater surge travel. Due to the minimal preload force, the piston will not loiter in the vicinity of the cylinder end, so that for a given muzzle energy, the stiff spring will have to generate higher pressure and temperature.
Another factor that affects spring stiffness is the diameter of the coil, and stiffness varies inversely as the mean diameter to the power of three. Most airgun mainsprings are in the region of 19-21mm (most are 20 point something mm), so the limited range of diameters means that it has nothing like the effect on stiffness of the wire diameter.
The third factor that determines spring stiffness is the length of the wire, taken as the number of coils, and the longer the wire, the greater the number of coils, the less stiff the spring.