Not 100% true.Xen said:the High-Cap M mags have the same bottom as the S mags.
This all depends on the caliber. Here's the lowdown:
Hi-cap (14 rd.) M9, 10rd. S40 magazines, and 12 rd. M40 magazines have the same baseplate. It is basically a flush, flat plate.
10-round M40 magazines have the same baseplate as S9 magazines. Medium-sized baseplate.
Standard (10rd.) M9 mags have a large baseplate that is not shared with any other magazine.
Here's a photo to better describe the sizes:
From left to right:
S40 magazine (modified to hold 9mm - more on that in a minute)
M9 hi-cap magazine
As you can see, the S40 and M9 hi-cap share the same baseplate. The S9 mag has the medium baseplate, and this is the same as the plate on an M40 magazine. The M9 mag has the tall baseplate - not shared with any other mag.
So the regular & hi-cap mags only share the same baseplate in .40 caliber (and the hi-cap M9). The hi-cap M9 plate is not the same as an S9 plate, so in 9mm it is not a direct swap.
As for the modified S40 magazine: I couldn't even FIND an S40 mag to buy, so I bought an M40 mag, and swapped baseplates with an M9 hi-cap to "create" an S40 magazine. I then bent the feed lips on this new mag, and it can hold 12 rounds of 9mm in my S9.