If you have a perfect airseal, the Modify S110 spring will put you at 395-400 FPS. Evey gun (300-400mm barrel) that I worked on and installed that spring gave me exactly 400 fps. You might get a different performance with it. But like the other members said, upgrade your other internals to handle that spring and a good bucking and barrel will give you the range. Installing a heavier spring doesn't mean your gun is performing better.
