Interesting experiment at the range today

[scattershot]

I have both a M&P 40 and a M&P 9, both Mod 2. Today as an experiment, I loaded the 9mm mag with .40ammo, and shot it without a bobble. I didn't change the followers or anything, just loaded up and shot it. I was surprised that this works. Anybody else try it? Gonna try 9mm in the .40 mag next time out, to see if that works.

 

[Boogs]

Would definitely appreciate a photos comparison of the mags next to each other broken down

 

[scattershot]

Didn't shoot any, but hand cycled a few 9mm rounds through the M&P9, using the .40 magazine. No hangups, but I got the impression that the rounds weren't as positive as the other way around.

 

[Saudade]

If you measure them, you'll find a difference in feed lip width and other dimensional differences in the followers.

9mm in the .40 mags may pop out.

So do they work interchangeably? Yes they do. Do they work reliably? You be the judge.

There have been posts in the past about this a few years back.

 

[Engineer1911]

On my Dillon 550 the 9 MM shell plate works with 40 S&W brass using 38 Special pins. I'm happy.

 

[1sailor]

Back when I had a gen 1 M&P40 I used to swap out the barrel for a 9mm barrel and the only issue I had with magazines in either configuration was that when using the 40S&W magazines with 9mm the slide would usually lock back when there was still one round in the mag. Otherwise I found them to be interchangeable. Same with my Beretta.