My MK40 Elite came NIB with one five round and one extended six round mag.
I bought a second six round extended mag.
Have ran literally thousands of rounds through the three with zero problems.
Have no experience with the polymer Kahr guns.
The MK40 mags are stainless steel.
Just like the frame, slide, barrel, trigger...
I found the Pearce extension this afternoon. Thanks. I ended up filing off just enough of the grip side of a P9 base plate to allow it to fit in place normally. I LOVE this little gun now, and suspect that my two P9s will spend most of their time in the safe.