If you take apart enough of the newer S&Ws with the Power Rib spring installed from the factory you will notice that the spring doesn't have near the arch as the old flat springs. It makes me wonder if S&W thought to use a longer strain screw to compensate for the rib or just used the same old part. I also think the round butt frame exacerbates this problem. I use an allen set screw w/locking compound from McMaster Carr and it solves a lot of issues. Of course this is after a careful stoning of the internal parts and a thorough polishing of the frame cavity.