My post this morning was in haste, so I just covered what happens in general terms. As has been noted by others the rebound spring does have a minor effect on the DA trigger pull. In addition, the mainspring tension also has a minor effect on the SA break. However, these cross effects are minor.
The way that I set my triggers is by adjusting the mainspring first to a weight that is about 1/2 lbs. heavier than my target. Then I'll install a lighter rebound spring that produces a single action break as close to 3 lbs. as I can get. With the SA sear on the trigger set to the factory angle I've found that in most cases a 14 lbs. rebound spring will produce a 3 lbs. break. After that I'll check the double action weight and refine it by either filing or shimming the strain screw until it's at my target weight.
BTW, I've tested for reliability using Winchester, Remington, Speer Lawman, and Federal ammunitions and 8 lbs. has proven reliable with all of these brands. With Sellier and Belloit ammo a heavier trigger may be needed, recent posts indicate this brand has an exceptionally hard primer. Unfortunately stock of S&B ammo dried up in my area about 2 years ago so I haven't been able to test for what DA trigger weight is reliable with Sellier and Belloit.
Now, about that target weight for the DA trigger.
With the guns that use a frame mounted firing pin I've found that installing a Cylinder & Slide extended firing pin will produce reliable ignition with a DA trigger weight of 8 lbs. even. This combination has also proven reliable in my 625 JM with the casings headspacing in the chambers, meaning without moon clips. As for why, the C&S firing pin is not only longer, it also features an extended travel capability in comparison to the factory firing pin. Now, one warning, this firing pin should NOT be used until the DA trigger weight has been reduced to 9 lbs. or less, using it with a factory weighted trigger runs a real risk of a pierced primer.
With those guns that feature a hammer mounted firing pin I've been more conservative. This system does not allow you to purchase a longer firing pin to assist with ignition reliability. Because of this I run my guns with a hammer mounted firing pin at 9 lbs. even and it's proven reliable. However, for an earlier revolver chambered in 45 ACP it might be necessary to increase the DA trigger to 9.5 lbs. just to insure it'll be 100% reliable. Since I don't own an earlier revolver in 45 ACP I am taking the conservative approach here and may be too conservative in how light you can take the trigger in a 45 ACP revolver.
Finally, concerning trigger return. One thing that I've found is critical in assuring a good crips trigger return with a lighter rebound spring is to pay attention to making the rebound slide as "drag free" as can be achieved. In older guns with a forged rebound slide that means breaking the sharp corner between the bottom and inside faces. The frame recess in these frames is milled and if the milling cutter had a bit of wear it can produce a small radius in that bottom corner in the frame recess. When this is the case, the sharp corner on the rebound slide can "dig into" that radius and produce drag. If you look at the newer MIM rebound slides you'll see that this corner is radiused specifically to eliminate Corner Drag. Next step is to smooth the frame recess a bit where the rebound slide runs, I like to wet sand the area with some 800 grit sandpaper until I see an even linear scratch pattern in the direction the rebound slide runs. Next, a light stoning of the rebound slide on a 1200 grit stone in the direction of travel will assure is runs smooth. Finally, I use a bit of Diamond lapping paste on a 1/8 diameter piece of bamboo skewer chucked in my Dremel to polish the bore in the rebound slide so that the spring won't drag. If you take these steps I expect you'll find that a 13 or 14 lbs. rebound spring will return quickly enough that most mortals won't be able to move their finger fast enough to lose contact with the trigger. However, if you have a problem with poor return, I'd suggest installing a heavier spring and doing a lot of shooting or dry firing to put some rub marks on the sideplate or frame recess to identify those areas that might need a bit more fitting.
Quite simply, rub marks are a guide that should NOT be ignored and I prefer to do my tuning after I run at least 1000 rounds downrange so I'll have some rub marks to guide me.
