I noticed that the DA pull on my 686 is very light compared to my 657. To me the 657 has quite a heavy pull and I have a hard time shooting accurately in DA because by the time I pull hard enough to move it it's hard to stop. The 686 I can keep under control and do better. What determines how heavy the trigger pull is in double action? The 657 is N frame, but I can't think of what that should have to do with it. Maybe an action job on the 657 could lighten it up?

There is great variance between the fit and finish of modern S&W revolver actions.

Some are a real mess, some less so.

You may also be experiencing a difference because of different frame size:

You experience the larger frame as harder to manage. Could be attributable to your hand interacting with the frame size and differences in the grip dimentions.

Could also have nothing to do with that: could be your N frame has metal shavings or other junk in it that was left there because the production quota didn't allow time to finish the job before the revolver got shipped.

Generally, the newer chronologically the revolver, the worse the fit and finish of the action.

You should also know that shooters often sought 'trigger jobs' in the good old days when S&W actions were uniformly much better than those seen today.

You may want to locate a knowledgable gunsmith to examine your revolver before you call Smith. At least then you'll know what needs o be addresses.

I checked a few others in my collection and all the large frame revolvers have what I find to be harder trigger pull in DA than the L frames. Even the performance center gun is harder. The 657 is the worst in my opinion because it breaks almost immediately after the hardest part of the pull, where the others seem to let off a little giving a place in the pull to finalize the aim before the break. This might be personal preference though, and I haven't spent much time working on my DA technique. My 41 and 44 both have target triggers too, which seems to make finessing DA more difficult.