A 10 round 40 mag holds 15 9mm rounds.
A 12 round 40 mag holds 16 9mm rounds (maybe 17, mine isn't broken in yet)
A 10 round 9mm mag holds 8 40 rounds.
A 15 (or 16) round 9mm mag holds ?40 rounds (I don't know, I don't have any. But I do have 18 40 mags, so I don't feel deprived! )
9mm mags work in 40 guns and 40 mags work in 9mm guns, sometimes minor feed lip mods are necessary.
40 mags and 357 Sig mags are the same except for the baseplates.
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