I just compared the length of the strain screw of my 686-5 with that of my 66-5 (both guns have a frame-mounted firing pin) and they are the same. I have the same reduced power mainsprings in these guns, but I have had no problems with light strikes in the 66-5.

Is it just that all these guns are a bit different?

Is there a way of getting a slightly longer strain screw for the 686?

I have never had a problem like this with my (many) older Smiths. These two guns are the only ones I own that have the frame-mounted firing pins and other new-fangled MIM parts.