I have two P320 compacts,  one in 9mm and one in .357 sig.  The mags from each gun will seat into the other gun,  this is a "feature" of the modular system.  

My questions is what would happen if I mixed up these mags at the range?  Would they load a round?  Would they fire?  Kaboom?  

Thinking of changing the base plates to a different color on the .357 sig mags to help prevent the mixup.

Interchangeable mags? Youmean like glocks, M&Ps and several CZs and I'm sure many others....

I don't think .357 Sig will chamber far enough in a 9mm to be able to fire, haven't tried it.


Pay attention to what you are doing. Right ammo in right gun etc.

Besides they are alreadu marked the mag bodies on the 9mm mags have indented lines run the length of them, 40/.357 are smooth.

If it makes you feel better, just get a paint pen and draw on them, would be pretty durable.

EDIT to add: don't mean to sound like an ass, just trying to point out that should be a minimal safety issue.

Paint pen sounds like a good idea.  Only glocks I have are in 9mm and .45 and they won't interchange,  I guess .40 and 9mm glock mags may.

They do.  The concern is shooting a 9x19mm in the .357" chamber, b/c it will chamber.  If the extractor holds the rim tightly enough, it will fire.  The case will probably fail as it has no support, and you might get a kaboom and a bullet stuck in the bbl w/ the same shot.  Figure out what happened, forget to check the bbl, load .357" correctly, and real KABOOM on next shot.

I have two P226's, one in .40 the other in 9mm.  Red electrical tape on the magazine bottoms have prevented me from being stupid.

9x19mm in a .40" S&W will probably go bloop!  Need to try that now that I have 9x19mm Glock mags.  

I'd change the grip module color one of them too...just for a little extra differentiation.

You'll get a case rupture and a failure to eject.  The bullet is not very likely to get stuck in the barrel.

The .357 is fde and the 9mm is black already.  Thinking about trying to find fde baseplates or just paint the ones i have.  Any sources for fde baseplates or fde paint to match?  Thanks.