Finally got to shoot a 648 I just bought couple of weeks ago. Group sizes were decent for my bad eyesight but one out of every cylinder full would not light off. Seems to be light strikes....very light dents in the rim. Not limited to one chamber....random among all 6. I did not break it down for a good cleaning before going to the range so maybe that's it. Will give me a chance to check the springs too.

Tighten the strain screw / mainspring. It is designed to be tight and I bet your problems goes away.

Update: strain screw was tight. Internals were spotless. However, on closer inspection I found some peculiarities and I am wondering about the firing pin. It does not seem to protrude far enough and it does not sit in the hole vertically.....it is canted to the right (about 2 o'clock). On a couple of chambers, I can see impact marks at the edge of the chamber that are also not at the 12 o'clock position.....on the other chambers there are no marks at all.

Really has me wondering if this thing was dry fired a lot...to the point of firing pin damage.

Could be, I've repaired plenty of .22's that some dingaling has dry fired to death.

Provided I can find one, how difficult is a firing pin change? It does not look hard but looks can be deceiving.

Yep - dry firing a rimfire is not good for the gun and can cause the problems you indicate.

I have yet to change a S&W frame mounted firing pin. If you're not sure you can do it I'd recommend you take it to a gunsmith.

Turned out to be pretty easy. Brownell's had the firing pin. Old one came out easy. The rimfire pins are keyed in the bushing and the spring keeps it from dropping completely down in the bushing so by far the most difficult part was keeping it all lined up going back in. Machine marks on the face of the bushing and frame made lining up the cross pin easy. Now for some time at te range.

Thoughts on the oil and polishing compound trick to slick up the internals?