I've got a P226ST I bought new sometime in the 2004-2005 time frame.

I've been carrying this a fair amount and really like the gun, but I'm wondering if it's possible to modernize this a bit by having someone mill the slide to accept one of the red dot type optics.

I've no idea WHAT specific optic I'd like to go with yet, so this is more me trying to figure if that would make sense since I already own and really like this particular gun, or if it's either cost prohibitive or not possible on an older slide and I should find a more modern production one already set up for optics.

(If it's both possible and not cost prohibitive, who do you like for that work?)

Considering the 'rarity' of the P226ST, I would recommend going with a new, optics ready slide so as to not decrease the value of the ST model. I'm no expert, but that's just me. Sig Arms website has some for sale with and without the RDO included. If you don't like it, you always could sell it here and still have the ST intact. Best wishes on your endeavor! I just installed my RDO on my Canik and will be trying it out soon.

Possible? I can’t see why not.
Many years ago I had a stainless steel 357/40 P226 slide milled for a Docter sight. It removed the rear sight and exposed the spring of the safety lock mechanism, but the latter didn’t matter with the sight installed.

I've had a zillon 226 slides milled. Older folded ones are a no no, but any modern one is fine. But not a chance I'd mill a 226st slide given the limited production numbers of the underlying pistol. Easy enough to get a new slide. But that's just me.

Order and wait for it to show up. They don't charge your card till ship. You will spend around the same machining and buying new optic, plus as been noted before you cut up a slide on a unique model. Also depending on how Sig machined the slide you my not be able make an optic cut without creating an opening in the slide.
https://www.provenoutfitters.c...assembly-w-romeo1pro

Thank you all for talking me out of cutting up my existing slide.

One more stupid question, but does a new slide require any kind of special fitting or anything, or just slap it on and go?

It’s usually possible to switch slides among different specimens of the same model SIG. As I recall, though, when I tried putting a stainless P226 slide from a gun with aluminum frame onto my P226ST frame it didn’t fit because the frame rails were too thick. If I remember later I will check to confirm that my memory is correct.