This Valken AEG main spring replacement fits any version of Airsoft AEG gearboxes. Its made of non-stain, extremely durable steel. The wire tension is precisely measured to ensure the best possible performance. The estimated FPS is 420-550, though the actual FPS may differ from the estimate depending on the alignment, power output, installation, inner barrel, cylinder, and more factors. The spring is designed to reduce the stress and tear of your gearbox.