This Prometheus MS100 AEG spring is, as Prometheus calls it, a non-linear spring. In other words, it’s a progressive spring, meaning it consists of coils that are wound close together and coils that are wound more far apart. The advantage of a progressively wound spring is the fact the teeth of the piston and gearbox components will take less of a beating as torque is gradually built up.
To start with, the load is light and will build up as the close wound coils are binding and the more widely wound coils are taking over, building up resistance in the last part of compression, just like a linear spring would do over the complete length.
Besides the fact the Prometheus MS100 AEG spring enhances the lifespan of the internal components of the airsoft replica, an added advantage is that a powerful motor gets the chance to build up momentum for a quick last part of compression (the heavy part), helping you create a higher rate of fire.