Smoothest (and lightest) is a 1920 vintage M&P (.38 S&W Special) bought new by my father-in-law (in 1920), and carried daily in the hip pocket of his overalls for 50-60 years. It never had any sort of action job unless you count the replacement of the stock rebound slide spring with one apparently selected solely on the basis on the fact it would fit in the hole. My best guess is this was done by someone named Bubba---address unknown.
At any rate, there came a time not too long ago when I decided to work on my close range, double action, rapid fire, point and shoot. In view of the fact I was not even remotely inclined to use any of the guns in my collection, I selected this one. The action was terrible---easily the worst I've ever experienced, and the first I've ever measured. The double action trigger pull was 22 pounds. I wasn't quite sure what it should have been, but damn well knew it should be less than 22 lbs.
So----given the options of covering it with money and sending it off to the pistolsmith du jour, slicking it up myself, or popping for a $20 spring kit, I chose the latter----and it was $17 something delivered (from Brownells)---and it was Jerry Miculek's kit.
The kit comes with instructions. Now I've been fooling with these things for more than 60 years, and damn well don't need any instructions to swap out a couple of springs---and the gun would not function when I was finished----or thought I was finished. It turned out the instructions were rather interesting----and you can set things for whatever pull weight you want----and if you decide you want a heavier pull later on, just get a new strain screw, and start over.
I set mine at 7 lbs. (the lightest recommended---and with the proviso you use Federal ammo/primers) I used WW ammo because I didn't have any Federal, and so far, so good---no failures to fire.
I took it to my gunsmith for show and tell. His comment was the same as mine: WOW!!
Ralph Tremaine