640 joy:
My 640 has a nice smooth trigger pull, much lighter and smoother than stock. First thing to understand is that Smith & Wesson designed and springs their revolvers to function under the worst possible conditions. That is sub freezing, NO maintenance, no lubrication etc. AND there are at least 23 lawyers inside every pistol. That said, a lot can be done to smooth and lighten the trigger pull.
The rebound slide was polished & smoothed as was the frame and side plate where the slide runs. The inside of the rebound slide was smoothed. (it was quite rough)
A lighter rebound spring was fitted I believe 13 lbs and a lighter hammer spring was fitted, light grease applied inside and outside the rebound slide. The internal face of the trigger where it meets and slides on the hammer lever was lightly polished as was the face of the hammer level where it met the trigger. All areas where metal slides over metal were smoothed and polished. Please note! NEVER NEVER REMOVE ANY METAL FROM ANY HAMMER OR TRIGGER ACTUATING SURFACES! Those parts are surface hardened and the hardened surface is very thin. Polish only and then lightly. Again light grease is used on contact surfaces of trigger & hammer.
Things NOT to do: Don't use the hammer spring strain screw to reduce trigger pull, change springs. If your Smith is double action/single action, never touch the single action contact points. (far too easy to mess it up badly)
Things to do: Use only high quality springs, lubricants etc. Wolff Gunsprings and Brownells action lubricant & Mobil 1, 10 wt. work for me.