I've been having a hell of a time with ftf's after lightening my glock 35 slide. Cut is very similar to zev dragon fly. I'm shooting factory ammo high pf. I've tried captured 13 and 15 pound ismi springs with no success. I've also tried uncaptured 13 pound round wire spring, which I think is working. What's the consensus of which spring works. Trying to avoid buying a whole set of springs to figure out the one that will work.