I bought a 40 S&W magazine on Ebay. It arrived and the package said 40 S&W. I opened the plastic container and pulled the magazine out. It has 9mm written on the follower and 9mm on both sides under the S&W Insignia. I wrote the seller and he said Smith and Wesson routinely interchanges parts on magazines it should work fine...
So, Is this true? will the 9mm magazine work in my 40 S&W and ammo? Is Smith and Wesson's Quality Control That bad?
I mean the container says "MP S&W 40" the magazine inside is clearly 9mm.
any help or answers would be appreciated.

In short.......NO.

The magazine opening on the 9mm is much smaller than that on the .40.

The only round that .40 cal mags are compatible with is .357 sig because it is a tapered .40 cal cartridge.

You can throw 9mm rounds into a .40 mag and they will fit standing still but bump it slightly and they will fly out like confetti. Also if you put .40 cal rounds in a 9mm mag they will either not fit, stack incorrectly because of wrong follower angle or they will stick inside the mag and refuse to feed.

Prying the lips out will not work either because the rear of the lips wont separate which will cause severe nosedive if a .40 round is put in a 9mm mag :-)

Sorry. It looks like the "seller" swapped mags and took you for a bit of a ride, I would request a .40 cal mag or a refund.

Thats about right. Thanks

Best way to tell is by the stamp. If it says 9mm, .40 wont fit. If it says .40 9mm will fall out haha. When in doubt. Put a round in it. If it doesnt seem PERFECT. Its not right haha

The good news is you can probably sell it for much more than the 40 versions...

This is true, 9mm are harder to find that .40"s

Give the seller a call & tell him S&W says that very definitely 9mm won't work in a .40 mag & vice versa. I'm sure S&W uses some of the same parts for all their magazines but sizing will still be different. As I understand it S&W magazines are manufactured by an outside contractor. Somewhere on one of the guns sites I'm on showed how to modify .40 cal to work with 9MM. The first attempt he broke the ear off of the opening. He had to heat it up with a torch to bend it enough to make it work. I'm sure

If you wanted to try it do it at the range if you can even get the ammo to fit but never use that mag for SD even if it works.

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Former USN, Retired US Army