I'd be more worried about side to side play when closed. Can't say much to your fore and aft issue (a little endshake is normal in a Smith) It might be that your retention screw on the frame above the trigger is loose, or too short. It's not a dangerous issue but not perfect either. Olf Fuff will be along sometime to set you straight.
I have a model 19 which has a very very slight crane play if I pull and push back and forth while the Cylinder is open. Is that normal?
ust for the Halibut, take the two dome-head sideplate screws out and exchange them.
If the one you take out of the hole behind the trigger-guard seems to start getting tight in the front crane retention hole before it is supposed to get tight?
Stop and put it back where it came from.
If it doesn't, tighten it and see if the crane will still open smoothly, and if the endplay is now gone.
rc
The frame is not the weak link.
The barrel forcing cone has a thin flat spot milled out for cylinder gas ring clearance.
That is the weak link, if there is one.