I will add that buying Kuhnhausen's book on S&W revolvers, the right screwdrivers, a small fine ceramic stone, some light grease (TWB25) and SLOWLY smoothing the moving parts inside will make the trigger much smoother and better.
The trigger pull on my 642 was over 12 pounds. It had a rough spot in the middle and a catch right before the hammer release. Now the pull is a little over 11 pounds and so smooth no one believes it's that heavy a pull. No spring changes.
Do NOT touch the actual hammer/sear contact points except with a tiny dab of grease.
Do go very slowly and don't try to get it all done at once.
With an improved trigger pull, I can shoot much more accurately with this little revolver. Kind of important for your primary carry piece.