Rastoff, I was about to do this (40 barrel in 9mm slide), but I took a 40 shell, and tried to fit it into the breech of my 9mm slide. It will slide into the space between the ridge and extractor from the bottom, but not from the top. Sliding in a 40 barrel into the slide, and you can get it to seat ok, but the edge of the brass base just barely allows it to fit.

What this means is that any slop in the chamber to slide orientation may allow the brass edge to strike the ridge on the breech, and either deform the brass, or not allow it to go into battery.

To do this, the breech itself needs some widening.