I used some fired brass to make my own for dry fire practice.
I used
empty .38 shells, primer removed, and forced o-ring rubber into the primer holes.
Cut the rubber flush, and done.
It works great, and seems to last well.
How much firing does it take to smooth out the action on a new 642?

I used some fired brass to make my own for dry fire practice.
I used
empty .38 shells, primer removed, and forced o-ring rubber into the primer holes.
Cut the rubber flush, and done.
It works great, and seems to last well.
How much firing does it take to smooth out the action on a new 642?

In years past, folks recommended putting silicone putty into the primer hole.

Like most Smiths, it should keep getting better for the life of the parts. But for me, 50% of the improvement seems to come inside of 500 cycles, next 25% in the next 500.
YMMV

I made some .327 snap caps replacing the fired primer with pieces of an eraser, I think the correct o-ring is a better idea.