AirsoftPro Spring action SVD CO2 upgrade set. The only way how to extremly increase your FPS up to 700 FPS (over 200 m/s).
Link where to buy:
http://airsoftpro.cz/en/dily-zbrani-upgrade/dily-pro-odstrelovaci-pusky/pro-manualni-svd/co2-upgrade-set-pro-manualni-svd-3.-generace-detail