S&W does tweak the 625JM - and other revolvers - by grinding the bearing face of the strain screw. This shortens the screw - and can leave a sharp edge to go into the hollow rib of a Wolff ribbed mainspring. Whether a full power or reduced power spring, this lessens the pre-load on the spring. Sure, the trigger feels even better - but the strikes will produce some ftfs - even, in the case of a lightened spring, with Federal primers. This was the case in both my new 625JM and my new 627 Pro. A new full-length strain screw - gratis from S&W - fixed that in both cases.
I elected to use a reduced power spring in my new 617. I took another fellow's advice - bought 8-32 socket head SS set screws (2/$.56 at Home Depot!). I ground the end flat, cleaned the hole and screw with alcohol, and coated the upper part of the set screw with blue Loctite. After solidifying, I screwed the screw in until the trigger felt similar to other K/L/N frame triggers in DA - using snap caps, of course. That did it - works great and pops el-cheapo .22 LRs day in/day out.
So... the problem in ftfs may just be the infamous strain screw. But... a C&S 'extended length fp' may have fixed it - what gives? The C&S fp is the same size - it's notch is wider, permitting more range. That also means it can flatten the little fp return spring more. Dry fires should be avoided, unlike with the OEM fp. In my book, the strain screw is the fix. Yeah, they do occasionally get 'short' fps... get a new one from S&W in that case.
Stainz