FWIW, I opted to have the slide milled on my VP9 (locally), mounted the optic directly to the slide and haven't looked back, as it were. I know it's not a USP but personally I consider whatever I'm carrying should be a functional, practical tool first.
I can see the front sight (Truglo) through the window (Holosun) but not able to index F-to-R sights. I was having a slower time aligning them anyway due to aged eyeballs, so that issue made little difference to me.
Also got a Dara IWB holster pre-cut for optics but had to send it back for re-cut because the optic sits on the slide, not a plate and the pistol wouldn't fully fit into the holster without pushing the slide it of battery .25". Dara sent me a shipping lable and I had it back in less than a week.
Also, shooting a pistol with an optic was a whole different way of sighting. Had to break the years old habit of one-eye-closed and also learn to perfect my grip before bringing the sight up to my dominate eye not the middle of my face. Sort of different because I'm left-eyed and right-handed.
I guess what I'm getting at is it's a steeper learning curve than I expected.
Sorry, got off track here. What were we talking about?
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I'd give my right arm to be ambidextrous.