I've heard of people using .40 s&w mags and loading them up with 9mm cartridges and getting extra rounds out of them for the XD's and Glocks. I live in NJ so im limited to 15 rds. I just purchased a M&P 9mm full size so besides the G&R Tactical 15 rounds mags that have been out of stock for a while now, i was hoping that someone tryed this trick for the 10 round .40 mags and maybe getting 13 rounds of 9mm in there.

That sounds like a scary chance to take that it would jam the firearm and/or not feed properly. Personally, I wouldn't even try this, but maybe some others that have much more experience than me can chime in and answer your question with personal experience.

But as for my opinion, you couldn't even pay me to take a chance of doing this and having a double feed or some other issue due to the round not feeding properly from the magazine or even getting jammed inside of the mag itself and rendering the mag useless when a susp/perp is coming at me. Uhhhh...no thanks. Not worth my life to take that chance just to get a couple of extra rounds in there. (again, not to me..YMMV)

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Mat - M&P 40c & 40 Shield

The base rim is only .020" smaller on 9mm so I can't see getting 3 rounds in maybe 2. Like said, the feedlips are different so while I might use it for the range I wouldn't trust it for self-defense. I have used the Taylor Freelance mag extensions, they will work with the stock spring but usually won't lock the slide open on a empty mag unless you install a longer spring. They'll add 3/40 or 4/9mm easily.

I've seen it done on the range.
I've also seen it jam on the range.
Just trying it for grins on the range? Why not?

Doing in a carry gun? Why risk a jam?

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Science plus Art

I never tried it on a Glock, but have done it with a BHP. Feeding was fine with a NEW BHP 40 mag but was sproradic, at best, from an older 40 mag. When KelTek first came out with the 40 conversion upper for the P11 9mm, the magazines where interchangable with just changing the mag follower. They had feed problems until they developed a dedicated 40 mag.

Class III

I know of folks using .40 ammo in 9mm mags successfully but not the other way around. Capacity is less however.
Randy

Some on this forum have put a 9mm stock barrel on a .40 and then loaded the stock .40 magazine with 9mm rounds. The action would lock open with one round left in the magazine due to the size difference, but otherwise worked fine.

As far as I know 9 mm bullets can feed a 30 Luger mag (7.65 mm Parabellum) or viceversa.
I agree with the opinions above that any different mag can jam.

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Salu2 a to2
Bizio

I have tried it with a Beretta 92 & 96. Mags worked fine in both guns.

I remember several years ago, some gun company was making 40 & 9mm handguns and were using the same mag. When you pulled the mag out, it had 9mm on one side and 40 S&W on the other with the holes drilled in proper place on each side.

I cant remember for the life of me who was making them though..

I have not tried it ijn a Glock.
I did "convert" a Sig 229 Sport in 40 S&W into an IDPA gun using a extra standard slide and Barsto 9mm conversion barrel. The unit ran well with 9mm using the 40 S&W magazines an d I was able to place fifteen 9omm rounds into the 12 round 40 S&W magazine.
I have also done the reverse. I purchased a CZ-40B in 40 S&W and for a long time it was impossible to purchase reasonably priced magazines. I used some old pre-ban 15 round 9mm Triple-K mags from my CZ-75 and loaded them with 40 S&W. The CZ-40B did not object.

I'm surprised someone hasn't rigged a 100-round mag together for range use.

Lop off the base of one mag and weld on another, then another, then another, etc. Find a really long spring. Have a 4 foot long mag HAHAHA