For my ICS I've purchased two different Guarder springs, one SP120 and one SP130, since I wasn't sure which one would let me near the 450 FPS limit I have in my field. The problem is that neither leave me there. The SP120 gives 415 FPS and the SP130 470 FPS. Talking with the store clerk he told me something like adding shims or something like that to the SP120 in order to reduce the space it has to compress and thus gain some FPS in the process. Has anyone do or find anything regarding this issue? I could also cut the SP130 a little, but I'm no expert and I'm not sure how much should I take. Also I believe the end result might not be as good as the whole thing.