I tried it. The barrel fits fine, as I expected. However, I tried manually cycling a few rounds through it. Extraction of the 9mm brass using the .40 slide and 9mm barrel was iffy at best. I doubt it would work very well, if at all, in actual firing. I would say you need a complete top end, not just a barrel. I have interchanged top ends in 9mm and .40 without any problems. My understanding is that the 9mm and .40 frames are exactly the same.
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