I bought a Wolff spring kit for the 642. I left the factory hammer spring in and installed the 14 pound rebound spring. With a whetstone I lightly polished the rebound slide and the sear surfaces, didn't get crazy with it just made 'em a little shiny. That done the trigger pull felt a bit more light and certainly smoother, although no one would mistake it for having had a good action job or even well broken in.
After work I put some rounds through the 642 the first time with the new rebound spring and a little polish work on the internals. The trigger doesn't feel dramatically more light but there was a noticeable difference in shooting. The sights don't get pulled off target quite as badly by the trigger pull, I don't have to think about it as much. I haven't done any real load development yet as its been too wet/dark to get out the chronograph. The revolver sights do seem to prefer stronger loads, I'd say S&W assumes you are going to use +P loads. Like other snubs I've owned the 642 is pretty easy to shoot in terms of recoil and muzzle jump. Don't really know why but that's always been my experience. Oddly, I find the 642 easier to shoot than the Charter Arms .32 H&R Magnum I used to have, you would think it would be the other way around.
