I have read on a few firearm websites that some people are using their .40 S&W magazines to shoot 9mm bullets. I wanted to see if it were possible, but only loaded a magazine. The 9mm bullets seem to fit, but just barely. I'm not going to shoot out this magazine, but wanted to see if it were true.
Has anyone here ever used a .40 S&W magazine to shoot 9mm bullets?

I don't see the point of doing so, since the .40 is a fatter cartridge.
I'd really have to be in a pinch to try to use my .40 mags in my 9mm.

During the AWB days I knew of a guy that used 10 round Springfield Armory XD40 magazines in his XD9, I think he could load 12 or 13 rounds of 9mm in the 10 round .40 cal mag. I fired it at the range once, it fed fine, no problems. Back when magazine capacity was restricted it gave smaller caliber guns a little more capacity but now that the Ban has sunset, I don't see the point. As long as the magazine feeds ok, there shouldn't be any problem.

I'm not going to try shooting the 9's from my .40 mag. I just wanted to see if they would actually fit without falling out. I have several 9mm magazines.
Just curious.

Of course, the Federal AWB and 10-round magazine limit is still in effect in certain loony moonbat states. When you can't find (or afford) a high-priced "pre-ban" 9mm magazine, sometimes it makes sense to buy a .40 "pre-ban" magazine and adjust the lips a little to make it work reliably with 9mm. It's been done. It's not ideal but it does work.

It should work with most mags.

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I may have missed it, but it was a factory mag, correct?

Where I've read about it being done is when folks put a 9mm barrel in their Shield 40's. Drop in conversion, from what I've read (can't go the other way, though, 40 barrel in a Shield 9). So using the 40 mags to feed it is cheaper than buying new mags.

We have 9 and 40 Shields, but have not tried the conversion stuff. No reason to, since we have both anyway.

Right.... Living in the gun friendly state of Kansas, I forget that there are still many places that restrict magazine capacity.

Disabled1, you sound hesitant to try firing your pistol with the .40 cal magazine; I wouldn't worry about it. You might have a failure to feed or some other jam but nothing catastrophic. Give it a try, at least you will know if it works or not.

That friend of mine never had a problem that I know of with his XD9 and I don't believe that he adjusted, "tweaked" or modified the XD40 mags in any way, he just used them as is. However I do know that he switched to higher capacity XD9 mags after they became available again.

Magazines CAN be designed from the start to work with both cartridges. I was at the range yesterday with a friend who has a CZ Shadow 2, whose magazines work with both calibers - there are different witness holes on opposite sides of the mags, marked for the caliber they represent. (He says the same mags work in the CZ 75-01, too.)

Other designs may or may not work reliably with both 9 and 40. So you just have to try each mag and see.

I regularly shoot 9mm rounds in some of my .40 to 9mm converted .40 Glocks using the .40 magazines. Works great with never a miss-feed.

Not all gun manufacturers .40 mags will work good in the corresponding 9mm guns as the round has to fit tight enough to stay put & must feed at the correct angle to work correctly.

I have a 10mm 1911 that actually feeds smother & better using .45 mags as the round sits a bit higher & at a better angle.

i know the magazines for a beretta 92 in 9mm will work in the 96 beretta 40 and the 40 mags will work in the 9mm as i have both and have swapped them out many times after i was told it would work, others may not work.

The force pushing the feed lips apart will be greater with 9mm loaded than the force from 40 being loaded in a 40 cal magazine. Whether it is enough to damage the mag is beyond me.