I have an S&W M&P/357Sig, full-size, first series.
I wish to put a Holosun HE507C-GR-V2 on it.

First question:
Has anyone experience with milling the 357Sig slide? The machinist who does this work on a lot of guns asked whether the S&W slide might be extra-hardened and therefore more difficult to cut.
He said there appears to be plenty of metal to allow the milling. I wonder whether the cut might weaken the slide. See question 2:

Second question:
Does anyone have experience with a holographic sight on the 357Sig? Some suggest the gun may be too rough on such sights for them to survive. As I asked above, might the cut weaken the slide too much?

No, if it's cut properly, the cut for an optic like this won't weaken the slide.

The Holosun HE507C-GR-V2 is not a holographic site. It uses an LED to create the reticle. A holographic site uses a laser and a special screen to create the reticle. With current technology, this type of site is too small to contain all the necessary electronics to support a holographic site.

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Rastoff: You're correct about the holo/LED. Olde Guye memory.
It uses wind power to keep the 40-volt battery charged, propeller hat included.