Some of the CZ75 .40 mags will work when loaded with 9mm. But not all of them. Can't remember which ones do and which don't, offhand. I bought a couple of each type to try, and one brand worked flawlessly, with no feeding issues whatsoever, holding 14 rounds of 9. The other brand I tried wouldn't hold them in at all - the feed lips were too wide. I'm pretty sure it was the Mec-Gar .40 mags that worked, as opposed to the CZ branded ones, but I'm not 100% certain.