If you want to decrease your velocity, you'd be better off getting a weaker main spring, instead of playing with valve springs. CCI makes three mainsprings red(strong), blue(weak), and silver(standard). If you want to play around with valve springs, you can get a madman cocker spring kit. Autococker valve springs will work as a nelson valve spring if you trim them just a little bit. The autococker mainspring is a drop in replacement for the nelson mainspring.