Will a M9 magazine fit in an S9?

The only difference is the size of the baseplate.

So yes, you can use any M9 magazine in an S9 (including hi-caps). It will stick out of the bottom of the gun about 1/2"

BTW - they're magazines, not clips.

you really only need one S magazine anyway.

I load up my two 14rnd M9 mags and my one S9 mag when i go to the range w/ the S9. Works great and w/ my meaty fingers the extra 1/2" sticking out the bottom makes it a more comfortable grip anyway.

The S just conceals better so having that extra 1/2" sticking out is not good for ccw. I never really take any extra mags w/ me when i ccw anyway, but if i did it would be the 14 rounder anyway.

I still have 4 10 rd M9 mags left if you're interested.