1.25" at the frame
1" on the slide
That is the measurements on my 9C so they are the same on the 40.
I don't think this is entirely true. My FNS 40 is a little wider at the top of the slide than my FNS 9. My FNS 9 is the same width as my FNS 9c at the top of the slide. I would say 1.25" at the frame and closer to 1.15-1.20" at the top of the slide for the 40.