If the correct approach is taken, there is no reason why a lighter trigger pull cannot be combined with 100% reliable ignition. The simple fact is the Performance Center has been offering this type of work for many years. In addition, when the revolver was the standard issue firearm for the Police nearly every one was treated to an Action Job consisting of 2 coils cut from the rebound spring and the strain screw filed down by 0.005 to 0.010 inch.
The key is to test as you go and do your testing with the most recalcitrant primers on the market. According to the Internet, that is currently CCI primers. So, get yourself a few boxes of Blaser Brass and find the point at which the gun just starts to misfire and then stiffen up the mainspring just a bit.
Please note, what I am posting is based on my experience with the K, L, and N Frames. I've never actually done any tuning on the J frames so my suggestions for this particular lockwork are based on my experience as a Mechanical Engineer.
Tweaking the mainspring tension is something that is easy to do on the K, L, N, and X frames because they all feature a strain screw that can be used to adjust the tension. Currently I've been experimenting with hand made shims that can be placed under the head of the strain screw so that I can adjust the strain screw in fixed increments as fine as 0.001 inch and have that strain screw fully tightened. BTW, using the factory mainspring currently both the 610 and 620 are 100% reliable with 0.010 shim shooting Blaser Brass, next step is to test with a 0.012 shim.
Unfortunately, the J frames use a coil mainspring so a different approach would be required, either adding a shim under the seated end of the mainspring to set the final tension or working with a pair of mainsprings and stair stepping them down in length by alternating springs. Once you find the point where it's short enough to cause unreliable ignition with a hard primer, you go back to the previous, slightly longer spring.
I'll also note that coil springs will shorten slightly due to cycling. Generally a coil spring will shorten by a fairly small fixed amount during the initial cycling and then "set" to a stable length which it will hold until it finally fails. As for why this happens, spring steel will work harden a bit from it's initial state and once it's achieved the maximum work hardened state possible with the alloy, it will stay at that hardness for life. The only exception is when the steel is continually cycled beyond it's Fatigue limit, a specific property of steel alloys. Since I rather doubt that the mainsprings on these revolvers are actually worked above the Fatigue limit, I don't think that comes into play. If possible, I would suggest starting with a pair of well used mainsprings because they'll already have taken their "set".
If you start with new mainsprings, make sure that you test fire the gun frequently for reliablity for at least the first 6 months. It would also be a good idea to have a fresh spare on hand that you can grind to length once you've determined how much the mainspring "sets" with use. I'll also note that any grinding done to shorten the mainspring be done "cold", ideally with a diamond tool. Small diamond burrs that fit a Dremel are availible from tool supply sources such as MSC Industrial Supply or Production Tool Supply and they work quickly on hard steels without generating much heat. Heat the end your working on until it's blue and you'll have annealed that end of the spring and the "set" will be much larger than with a "cold" ground spring. Note, the process here is to determine the minimum reliable length, make note of that length, then go back 6 to 12 months later and determine how much the spring has "set". Then take a fresh spring and cold grind it to the initial reliable length PLUS the amount of "set".
Once you find the minimum tension that will provide 100% reliable ignition, you can then address the rebound spring. However, I've found that the rebound spring doesn't have as great an effect on the double action trigger pull as one might expect. The difference in DA pull with a 12 lbs. rebound spring and a 14 lbs. rebound spring is neglibable, about 1/2 lbs. at best. Basically the trigger bar that operates on the rebound slide has a large mechanical advantage over the rebound slide, so a 2 lbs. change in the rebound spring will have a much lower effect on the DA triggger pull change. The good news hear is that the rebound slide has no effect at all on ignition. However it will effect how quicly the trigger returns in rapid fire and a very light rebound spring may mismatch enough with a heavy mainspring that the trigger won't return fully. When that happens, the lockwork will completelly seize up until you push the trigger fully forward.
However, the rebound spring will have a very noticable effect on the Single Action trigger pull. My experience has shown the factory 16 lbs. rebound spring will produce a SA pull of 4 lbs., a 14 lbs. rebound spring will produce a SA pull just a hair over 3 lbs., a 12 lbs. rebound spring will produce a SA pull of about 1 3/4 lbs. I've also found that a 12 lbs. rebound spring will cause the action to "tie up" on some guns during live fire when combined with a full power mainspring. Since 3 lbs. is the SA trigger pull I feel most comfortable with, the 14 lbs. spring is my "go to" rebound spring, it's proven to be 100% reliable for trigger return, has a good quick return, and put's the SA trigger at my personal sweet spot.
Now, one tip pertaining to the rebound slide assembly that I found a bit by accident when working on my 620 and 610 side by side. That is that the N frames use a small rod on the inside of the rebound spring that acts as a trigger stop. However, that stop rod also limits how much the rebound spring can "buckle" inside the bore in the rebound slide. Because of this I noticed that the 610 was much smoother when cycling the rebound slide by pulling the trigger with the hammer out of the gun than my 620, despite the bores of both rebound slides having been fully polished. Next time Brownell's has the stop rods in stock I plan on adding them to all of my K and L frames. Is it really necessary, not at all, however on a slow DA stroke my 610 is perfectly smooth from start to finish and I can feel just a hint of coil bind on the guns that don't have this stop rod. BTW, orientation of this rod is critical, get it in backwards and the lockwork will tie up with the gun held in certain positions. If that happens, flip it end for end.
Now for detail work on the gun. One area I would pay a lot of attention to is the mainspring strut. I would suggest stoning the edges that contact the mainspring with a 1200 grit stone until those edgest are perfectly smooth. However, be careful about removing the minimum amount of material while doing this. Reducing the size of the stut will permit the spring to buckle a bit more and if it buckles too much friction between the strut and the spring will increase, leading to a "scratchy" trigger pull. All you want to do is knock down any burrs, it's a case where less is best.
Rebound slide. Chamfer the sharp corner that rides against the inside lower edged in the recess of the frame, this will reduce "corner drag", which is actually that sharp corner trying to cut into the frame. Stone the frame when the rebound slide runs just enough to remove any high spots in the frame but don't try and make it glass smooth. The slight depressions left will act as a resevior for lube and keep the rebound slide running smooth. Stone the surfaces of the rebound slide that rub on the frame to the point where about 50-60% of these areas are fresh metal. Interior bore of the rebound slide should also be polished, I found that bamboo skewers of the correct diameter can be chucked up in a Dremel and allow this to be done fairly quickly with polishing compound, however it requires a light touch and if you see smoke it's time for a new piece of skewer and a bit slower RPM's on the Dremel.
Other recomended touches are a small chamfer on the DA sear on the hammer where if flips free of the trigger, on the late MIM guns this is a standard feature. You can also polish the DA sear surface on the trigger, however on a well used gun this will already have been done by simple firing.
Finally, all stoning should be in the direction of travel. It's also an excellent idea to observe the recess in the frame for any drag marks left by the hammer. If you see hammer drag, you'll need to correct this by adding a shim on the hammer stud to replace material that's been worn off the raised boss. Then it's a simple matter of putting some very light bits of lube in place for the hammer, rebound slide, and trigger pivot and closing everything up.
