A couple of years ago I bought a nice 4" 617-1 at a show. A previous owner had lightened the action -- a lot -- and the gun had a simply wonderful trigger. I quickly found out, however, that the price to be paid for superlight triggers on rimfire revolvers is unreliable ignition. I was getting FTFs on about 1 of every 6 rounds I fired. No, the problem wasn't a backed-out strain spring, I made sure that was tightened as far as it could go and I still got a lot of FTFs. For a while I put up with it, rationalizing that this was only a range gun and the FTFs were worth the ultra smooth trigger. Eventually, however, I tired of it and took the gun to a smith. He suggested putting in a slightly heavier mainspring. I did and the problem disappeared instantly. The trigger is still very light and wonderfully smooth but now I get 100% ignition.
The problem, as he explained it to me, is that rimfires simply do not ignite as easily or as reliably as centerfire ammo. Thus, there needs to be a little extra "oomph" on the hammer drop to assure reliable ignition.
As corroboration of that I called Smith and spoke to someone in their repairs department. He told me that Smith will not do a trigger job on a rimfire revolver precisely because lightening the trigger will often produce FTFs. He said that it was Smith's judgment that a lot of factory .22 lr ammo is just not all that reliable absent a pretty heavy hammer drop and, so, Smith deliberately made the triggers a bit heavier on their .22s and didn't do action jobs on them.