I have a 642. So far, I have installed a Wolff 8 lb mainspring and 14 lb rebound spring. I have both 13 and 12 lb rebound springs on order, as it is still a bit too stiff for my liking.
If you go this route, be sure to grind the ends of the rebound spring flat, as is the factory rebound spring. Otherwise, it is difficult to install even with a dedicated gunsmithing tool. Grinding is easy with a common drill and grinding stone attachment.

I would advise others to be very careful with any power tools and abrasives.

You can get a nice smooth rebound slide, bottom end edges included,
with a fine polishing stone, in just a few minutes.

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Ogy

I recommended grinding the ends of the Wolff spring, not the slide.

Sorry about that!

I never heard of that being recommended before so my mind must have reverted to what's "normally" done.

With that said, I usually put my springs in a vibrator / tumbler for a while with corn cob media to polish them up a little.

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Ogy

The factory rebound spring has the ends ground flat, while the Wolff springs do not. It isn't *too bad* trying to install the Wolff 14 lb rebound spring, but for lesser weights the length is greater (more preload) and they are difficult to compress without them bending and going 'boinnngggg' across the room. Grinding the edges flat makes them alot easier to compress and install. That's probably why the factory does it.