On the 617 with the standard firing pin, IMO the only harm that dry firing will do is stress the hammer. The fact is the firing pin doesn't extend far enough to contact the cylinder so the only real harm is the shock that the hammer strike causes. There isn't anything present to cushion the hammer fall when it falls so that it shoots the firing pin forward and then hits the frame, so it's a heck of a shock. I seen reports of hammer spurs breaking off due to excessive dry firing and suspect that it no applies to any S&W with a frame mounted firing pin.
Best way to avoid this problem is by using snap caps to act as a cushion for the firing pin and hammer. However you cannot find snap caps for rimfires, so in this case it's best to save your fired brass and use an undimpled area on the rim for each dry fire strike. It's a big PITA but it will protect the gun. BTW, if you compare the noise with an undimpled area under the pin to one that has been struck, you'll note that it's distinctly different.
BTW, I prefer the 6 shot 617. Not only is there less to clean but it's also less time waiting for smoke to clear. I do most of my shooting at indoor ranges because outdoor rages in my area are a minimum 35 mile drive. One thing about shooting 22LR in a revolver is that it's a very smoky shooting round. So much so that I have to wait for hte smoke to clear after 6 shots so I can see the target. With my 6 shooter, I'm reloading at that point, so the smoke has a chance to clear. With a 10 shooter it would probably mean shooting 5 at a time with a pause as long as a reload takes to wait for the smoke to clear. So, for me, a 6 shooter is a more logical choice. Plus, it's traditional and I like tradition.
