I do not have any of the longer mags, but I have successfully used 9mm ammo in the regular USP, USPc and P30 .40S&W magazines.
I am sure immediately there will be ten replies, doing that is wrong, you should not risk your life, always use only factory magazines and so on.
That is precisely why I am not saying anyone should do anything. I am simply sharing my experience.
In my mags, I am free to store chewed bubble gum, if I were so crazy to do so. :-)
Now on the flip side there are some .40S&W magazines, which do not work for 9mm:
- Walther P99
- Beretta Cougar
- Beretta PX4
- CZ P-07/P-10C (not P-09 and P-10F)