Looking at a used glock 19 gen 5 non MOS that looks like a guy had milled to direct mount a holosun 509. I don't know shit about pistol red dots. There does not appear to be an adapter plate already on the slide and it's not a MOS glock, which leads me to believe it's direct mounted. I have not actually seen the pistol in person, just a pic.

I may like the 509, I may not. Say I don't, is there an adapter plate to go from the 509 to a RMR footprint? The price is good enough that I am considering this pistol over just buying new.

No there isn't.

I would first confirm what the footprint is. If it is the 509, they’re good to go.

509 is a great optic

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I second the option to just buy a 509T. Love mine.

Is a slide milled to direct mount a 509 one of the more bomb proof mounting options?

OP, the 509T is a great optic!
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Direct milling for any dot will be the strongest method of mounting.

The 509 is solid, and a cut slide keeps it as low as possible which is good since it’s a larger optic. If you end up not liking it for some reason, getting a new slide with a different cut would be the easier and probably cheaper route, especially if it’s a Glock.

Also keep an eye out for Blem/open box sales on primary arms. You can easily save a couple hundred dollars if you snag one.

My favorite handgun optic: