As a rule the rebound spring supplied in new S&W revolvers run about 16 lbs. I've played around with different rebound springs and have settled on using either 13 or 14 lbs. rebound springs, mostly the 14 lbs. spring. It yields a single action break right at 3 lbs. provided the sear on the trigger has been left "factory". Currently I have 5 S&W's and all but one have a 14 lbs. rebound spring, the exception is my 620 which needed a 13 lbs. spring to achieve that 3 lbs. SA trigger which I prefer.
I've also tested this rebound spring with the mainspring at full factory power and with the mainspring reduced to provide an 8 lbs. DA trigger. While it may seem counter to common thought, the difference in the SA trigger break didn't change by more than 3 ounces between a full power mainspring and a reduced power mainspring. I've concluded that the mainspring has little effect on the weight of the single action break, that is determined by the rebound spring.
Early on I tried a 12 lbs. rebound spring in 2 of my revolvers, a 617 and a 610-3. Function was perfect for both guns in dry fire testing but in live fire the 610 locked up twice. In the case of the 610 I believe the lockup was due to the pushrod on the trigger that moves the rebound slide coming out of place. Pushing the trigger forward did NOT free up the lockwork, however wiggling the hammer back and forth with the gun unloaded eventually put things back in place. In the 617 that 12 lbs. rebound spring functioned perfectly. I suspect that the cause of the issue with the 610 was that there was just a hint of drag on the rebound slide, just enough to allow it to lose contact with the pushrod on the trigger, at which point the recoil jarred it out of place. After it happened I took the 610 home, did a bit more smoothing in the frame recess, stoned the rebound slide a touch more, and installed that 14 lbs. rebound spring. I also put the 14 lbs. rebound spring in the 617 at the same time. Fact is that I felt the SA trigger break was too light and wanted it a bit heavier.
As for that 620 being the odd duck, I suspect that it's likely a case of variance in the actual weight of the spring. However it's also possible the sear on the hammer is slighly angled in such a way that it increased the SA weight. What ever the actual cause, I simply experimented with different springs until I got the 3 lbs. trigger break I wanted.
As for function in terms of "outrunning" the trigger return, that hasn't happened with any of my revolvers. Trigger return is positive and fast enough that I've managed to get a .23 second split time with my 617, however that is simply because the muzzle on the 617 barely moves when it fires so it's a matter of pulling the trigger as rapidly as I am capable of doing. BTW, I still find it simply amazing that Jerry Miculek can actually move his finger fast enough to achieve a .12 second split time, that is well beyond what I'll ever be able to do. At that rate of fire I would expect that a heavier rebound spring would be needed.
Now, as for light strikes, that has nothing to do with the rebound spring. Power to ignite the primer is provided completely by the mainspring. I've also found that the DA trigger weight is effected very little by the rebound spring, that is mainly effected by the mainspring. As for why, the trigger move the rebound spring with what is best described as a "toggle action" and that means that as the rebound slide is driven back the mechanical advantage of the trigger increases rapidly. What it means is that if you want a lighter DA trigger you will have to reduce the power of the mainspring. If you are looking at a revolver with a hammer mounted firing pin I would suggest you target a 9 lbs. DA trigger. If you have a centerfire revolver with a frame mounted firing pin you can easily get down to 8 lbs. provided that at the same time you replace the factory firing pin with a longer firing pin from either Apex Tactical or Cylinder and Slide.
I've used the Cylinder and Slide Firing pins in my 620, 610, and 625 JM and there is one thing you should make note of. The C&S firing pins are not only longer than the factory, they also feature an extended travel capability. Because of the combination of these features I recomend they ONLY be used AFTER the mainspring has been reduced to produce a maximum DA trigger weight of 9 lbs. I tried the C&S firing pin in my 610 when the trigger was still at the factory supplied 10.5 lbs. and I saw that a full 30% of the primers in my ammo had cracks in the firing pin dimples and were darkened by gas leakage. Basically, it's quite possible that you could pierce a primer with this firing pin if you don't lighten the trigger. You also MUST use a snap cap for ANY dry firing with this firing pin because the travel slot is long enough that it will wad up the firing pin return spring if you don't.
Now for the good news. With this firing pin I've been able to tune the 610, 620, and 625 to a DA trigger of 8 lbs. and have not hand one single misfire. I've also tested for the possiblity of "marginal ignition" effecting accuracy and have seen no change in accuracy. Ammo's used for the reliability testing was Remington UMC, Federal Champion, American Eagle, Speer Lawman, Winchester White Box, and Winchester Ranger-T. If I can ever happen on some Sellier and Belloit I plan on trying that because it's recently developed a reputation for hard primers, however it's currently unavailable in my area. Basically, all three revolvers have proven 100% reliable with every brand of ammo I've been able to get my hands on. BTW, with the 625 I've tested for reliability with and without moon clips and it's perfect.
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