Other than the markings and the witness holes the P229 9mm mag and the 40 S&W look identical.
Are They ? In other words can the 9 marked mags be used to shoot 40 in a 40 P229 ?
Thanks in advance.

If you’re referring to “serious” purposes such as defensive carry, the stock answer is of course that we should always use the proper magazines. For practice/range use, though, your question prompted me to conduct a (very limited) test using an old version 13-round 9mm P229 magazine and 40 S&W cartridges in a 357/40 P229. The rounds were inert but duplicates of live training cartridges with FMJ bullets and brass cases.

When five cartridges were loaded in the 9mm magazine and manually cycled through the pistol, there were no failures to feed, chamber, or eject.* I therefore believe that the same would be true of live fire, although bullet style could have an effect. In addition, ultimately the only way to be certain is to conduct one’s own live fire testing with the specific ammunition that will be used. Plus my test was conducted with only one magazine and one pistol.

As a somewhat related issue, 9mm and 357/40 P226 magazines are not completely interchangeable. The feed lips of the 357/40 mags I tested do not hold 9mm cartridges properly and allow the bullet noses to tip up and out of position.

* And I will take this opportunity to make a point about terminology: “Feeding” is what the magazine does: i.e., push the cartridges up to where they can be pushed into the chamber by the slide, and that subsequent part of the autoloading cycle is “chambering.” Actual failures to “feed” are extremely rare and usually best corrected by discarding the bad mag. Failures of the round to go into the chamber due to any of several reasons from inadequate lubrication of bullet design are—strangely enough—failures to chamber.

The 9mm magazines do cycle the .40s&w cartridge fine. Found this out last week when I went to the range with a pair of 229s in 40 and 357sig.

I inadvertently grabbed a couple of the 9mm magazines when loading for the range. Didn’t realize it until I was back home and was putting the magazines away.

Like Sigfreund said, not sure if it would work the other way around.

Thank you both.

I have run 9mm out of several brands of .40 mags and never had issue (SIG/HK/CZ). That said I would never rely on that for actual carry or self defense unless I had no other choice. Never an issue at the range though in my albeit limited testing.

I would say not really. Always use the mag with the correct caliber mark. The P229-1 mag has the same external dimensions as the P229 40/357 mag but they are different internally. Notice how the P229-1 mag has tracks running down the sides of the tube. That’s because 9mm and 40/357 need different widths to stagger at the ideal 60 degrees. Those tracks make it so that the P229-1 mag has the same internal dimensions as the old P228 (or Legacy 9mm P229) mag. Springs and followers may also be different as well and then there’s the witness hole issue (you have to count the rounds yourself).

The P226 is a different story. Current mags from Mec-Gar are almost identical between 9mm and 40/357 variants, with the only difference being the location and numbering of the witness holes. But older ban-era mags aren’t like this (the 40/357 mags had wider feed lips to prevent someone using them for 9mm because more than 10 rounds would fit).