Lol just get something with the name ''m100 spring/s100'' xD Might be a bit off compared to prometheus but the fps are close.
http://www.airsoftparts.ca/store2/in...roducts_id=885
Even if your over you can just "work you spring" or take out your o-ring in your piston head if its over 350.
There are alot of springs in the market, find the m100 spring...