Also of note, .40 mags usually work for 9mm pistols, discovered back in the day when "hi caps" were banned (my favorite: cheap and easy to find 10 round .40 Firestar plus mags... but there is no .40 Firestar plus. They do fit 13 rounds 9mm just fine though... tricky MecGar!).
Anyway, they present the round a BIT higher than the 9mm mags. If anyone is playing hard with these sorts of conversions, and feels they are finding odd feeding from ramp angles with some rounds, try a .40 mag before you tweak feedlips, and see if it allows more of a straightline feed.
One note on steel mags in plastic guns: It will often wear down the mag latch, to catastrophic failure (either a groove that makes it lock up, or a worn edge that makes it not hold mags at all). At least be very careful to stone the cut smooth and break the corners. Inspect the mag latch periodically and if wearing, cut it out or get a metal aftermarket one.
If anyone wants to go crazier: I have mixed followers. G21 followers in a Megastar .45 was one. Extra round, integrates with the glock springs for easier availability, etc. No stoppages. Lesson is: mags are pretty constrained in size, a lot of stuff will interchange. Try it, anything might work.