I got my Glock 20 slide back from having it milled for an optic. After assembling I noticed the slide was not flush with the frame. It seems to lock up and function properly. Any ideas?
There's a typical variation with glocks in general. Here's my G20SF, with the slide sticking out past the frame. Everything is stock except the barrel.
If it had bushing, it could cover up all that "perfection" at the muzzle.
Really though, I would completely strip down the slide assembly and reassemble. Could be a small bit of anything knocking it out of sorts. Worst bunch of malfunctions I ever had was when some pocket lint somehow got into my G20 firing pin channel.
Have you tried it without the optics and the mounting plate installed. Maybe it was milled too deep and the screws are too lomg and are hitting inside the upper? Easy to try.