Spring strength isn't determined by length. I have some long springs that shoot sub-300fps and a short one that's in excess of 400fps. It depends more on the strength of the coils and the tension in them. A standard M100 in a short barrel MP5 should shoot at low 300s, unless you have already upgraded all the other bits in the gearbox. It will also change if you have bearing spring guides fitted to the piston head and spring guide base, as that will shorten the amount of space the spring has to fit and expand when under tension on the firing cycle. If you have already stripped the gearbox then fit the new spring and run two or three full hicap mags through the gun to bed in the spring, then get it chrono'd at the next game. And as PT247 said, set up the hop for 0.25s or 0.30s then chrono with 0.2s on this setting, as that should reduce the fps by about 20-30fps. Then play with the heavier BBs until the spring settles in and you can set the hop for your BB weight of choice, although 0.25s really ought to be the minimum weight you use for better accuracy and grouping.
