I have 2 NP22 mags from Marstar as well as 3 Sig 226 mags for my NP22. I had to dremmel the slot ever so slightly to ensure they caught when in the mag well. My calipers couldn't even measure how much I had to shave because it was so little.
Now they've been shaved, the 226 mags work just as well as the Norc factory NP22 mags.