There are 2 reasons for not going too light on the rebound spring.
One is that you may experience a sluggish or failed trigger return. If the trigger doesn't return fully it can completely tie up the lockwork. In addition a sluggish trigger return can slow the firing rate noticeably.
The other reason is that a light rebound spring will lighten the single action trigger pull. In a gun that a previous owner has stoned the sear on the trigger, a 12 lbs. rebound spring can result in a sub 2 lbs. single action trigger.
BTW, I've done some controlled experiments on the triggers of a 67-1, 617, 610-3, and 620. For a basic rule of thumb a 14 lbs. rebound spring will result in a SA trigger pull just over 3 lbs. if the sear on the trigger hasn't been messed with. A 12 lbs. rebound spring will take the SA pull to the region of 2.5 lbs. If someone has tinkered with the sear on the trigger, a 12 lbs. rebound spring can result in a SA pull of ony 1.5 lbs.
On the double action side, the effect of changing the rebound spring really isn't noticeable. I've experimented with rebound springs from 12 to 15 lbs. in my 620 and when measuring the DA pull with a Lyman digital trigger gage the change in weight fell within the range of variation for a series of trial pulls. The average weight may have fallen by perhaps as much as 1/2 lbs. but IMO anything lighter that 13 lbs. really isn't beneficial. As for why the difference is so slight, my hunch is that the position of the trigger relative to the position of the toggle link that drives the rebound slide masks the effect of a lighter rebound spring. Basically at the start of the trigger pull most of the weight is controlled by the mainspring acting on the hammer and at the end of the trigger stroke that toggle is so nearly straight on to the rebound slide that all you can feel is that fully tensioned mainspring. I was a bit surprized by this but after seeing the same behavior in 4 seperate revolvers I've come to the conclusion that the weight of the rebound spring has little effect on DA pull if your running a trigger over 7 lbs.
Now, Bullseye may take exception to this, however he's running guns with ball bearing lockworks and a sub 4 lbs. DA pull. On a gun with that state of tuning I suspect that a change in the weight of the rebound spring would have a measureable effect.
Personally, I prefer a 14 lbs. rebound spring over any other weight. With the exception of my 617, which needs to have the trigger sear sharpened, it's resulted in a SA trigger between 3 lbs. 2 ounces and 3 lbs. 4 ounces. That is right where I like my single action trigger to break.
As for the mainspring, all 4 of my guns are using the factory mainspring and I've done my adjustment for weight by shimming the position of the strain screw. I've also found that Target hammers actually require MORE power from the mainspring than Service hammers. While that seems counter to conventional wisdom, my testing has indicated that primer ignition is a function of Kenetic Energy, NOT momentum. Since velocity is squared when determining Ke, a lighter hammer moving faster can produce the same energy as a heavier hammer moving slower. Heavier hammers require more power to get up to speed, so you need more power in the mainspring.
Finally, there is more to ignition than just the round firing. I've determine that you can have the reliable ignition but see an observable loss in accuracy when you lighten the trigger in controlled steps. When I first took the trigger on my 620 to 8 lbs. the ignition was perfectly reliable with Speer Lawman, however the groups had doubled in size compared to what I saw when it was set to 9 lbs. I also saw groups that were vertically oriented, an indication of a variation in velocity. Fortunately, simply installing the extended length firing pin from Cylinder and Slide brought that accuracy back to where it was with the trigger at 9 lbs. IMO that is one distinct advantage to the frame mounted firing pins, it gives you tuning options that were lacking with the hammer mounted firing pins.
Now, to sum things up. Once you've bee "bit" by the tuning bug the urge to improve won't go away. I would advise that once you start tuning for a lighter trigger to plan on doing some Benchrest shooting for accuracy. Once you start to see the accuracy falling off, either go back to the previous weight or install an extended firing pin from Cylinder & Slide or Apex Tactical. BTW, I do NOT endorse using one of these firing pins in a gun that has not been tuned for a lighter DA trigger, when I tried the C&S firing pin in my bone stock 610 I saw evidence of primer leakage at a 30% rate and cracking in the primer dimples under magnification on those primers that leaked. Now that the 610 has been tuned to a 9 lbs. trigger, that primer leakage issue has gone away.
