[uncle_walty]

My M9 14 rounders works for my M357 without any modifications. The top round sits a little lower but still feeds fine. About 200 rounds so far only. 357 Sig base dia is same as .40 S&W. YMMV.

The M40 mags for the 9mm require the lips be bent in since the 9mm rounds can be pulled through between the lips if they're not bent.

 

[uncle_walty]

Using 40 S&W in M9 mag may work without bending the lips. If you get failures to pick up a fresh round or FTF, then you can try bending the lips out a little to let the top round sit higher. The 357 Sig may feed easier than 40 S&W due to the bottle neck. Note it is much easier to bend the lips out than to bend them in. Also take care at the back of the mag where the lips are supported by the little gussets, I cracked one side of that joint on one of my 10 round M40 mags when I was bending it back out for 357 Sig use after I'd convered it to 9mm before the full caps were available. That's when I decided to just try the 357 sig in the M9 mags without modification to see if it would work so I wouldn't risk cracking another one. The cracked one still works fine except I know the crack is there.