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M9/ M40 mag Question

2272 Views 5 Replies 3 Participants Last post by  uncle_walty
According to the USA Steyr web page, the M-series mags work for both 9mm and .40 SW: "This Magazine will work will the following calibers:
.40 S&W 12 Round and .9mm 14 Round".

Is that accurate? Can I load my M9 mags with .40 cartridges & then shoot them in a M40?
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When it was a problem to find hi-capacity magazines in 9mm, many shooters bought available M40 magazines and bent the feed lips down ever-so-slightly, and they would carry 12 rounds of 9mm. I guess you could open the 9mm magazine feed lips up a tad and use then in the .40, but I do not know of anyone who has done this.

P.S. .40 mags are used for the .357 SIG Steyr. If you have a .40 Steyr, you need only change the barrel to shoot .357 sig. Mags work interchangeably (sp?)

Regards, CGuns
Thanks. I just picked up a M40 to go with my M9, so I was hoping that I could save a few bucks and interchange the mags. I guess I'll try to modify a 9mm mag and see what happens.

My M9 14 rounders works for my M357 without any modifications. The top round sits a little lower but still feeds fine. About 200 rounds so far only. 357 Sig base dia is same as .40 S&W. YMMV.

The M40 mags for the 9mm require the lips be bent in since the 9mm rounds can be pulled through between the lips if they're not bent.
So if I want to put 40 S&W in a 9mm mag, I just have to bend the top out a little bit...right? Conversely, I need to slightly bend the top in to modify a 40 mag for 9mm?
Using 40 S&W in M9 mag may work without bending the lips. If you get failures to pick up a fresh round or FTF, then you can try bending the lips out a little to let the top round sit higher. The 357 Sig may feed easier than 40 S&W due to the bottle neck. Note it is much easier to bend the lips out than to bend them in. Also take care at the back of the mag where the lips are supported by the little gussets, I cracked one side of that joint on one of my 10 round M40 mags when I was bending it back out for 357 Sig use after I'd convered it to 9mm before the full caps were available. That's when I decided to just try the 357 sig in the M9 mags without modification to see if it would work so I wouldn't risk cracking another one. The cracked one still works fine except I know the crack is there.
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