Colt 9mm P mags have a block at the rear, and the lips are shaped differently from the .38 Super mag. The frames are the same and the .38 ejector should work OK. Note that the slide has to be 9mm, though. The .38 is semi-rimmed and the extractor won't fit quite right for 9mm. Of course all of this can be overcome with money and time. BUT, with all the 9mms out there, is building one worth it?