My preferred pistol, cartridge combination is the P229 in .357SIG. Since SIG has stopped making the P229 in .40/.357SIG, I have heard from a few people (who have questionable firearms knowledge) that a 9mm P229 can be converted to .40 or .357SIG with just a barrel and magazine swap (direct drop in , no milling). I could have sworn, somewhere on this forum, I read that it could be done, but that milling and fitment was required. I tried searching for the thread where I thought I read this, but I couldn't find it.
My current P229 has over 30,000rnds through it. It would be great if I could buy a 9mm P229, drop in a barrel, and continue to carry .357SIG. I have been trying out a FNX45 and have been pleased with carrying it, so far, but my P229 definitely conceals better, and I believe the projectile geometry of the .357SIG at velocity is a better option all around than the 160gr Barnes cartridges from Buffalo Bore I'm carrying in my FNX. I'm using the Buffalo Bore 125gr@1425fps or Speer GD 125gr for the .357SIG.
I apologize if this has been addressed and answered. I swore I read something about this somewhere, I just couldn't find it.

If I understand correctly, it sounds like you want to go from .357SIG to 9mm and not vice versa.

Bar-sto makes a conversion barrel that allows you to do this. Per their description you will also need a 9mm magazine and maybe a lighter recoil spring:

BAR-STO Sig P-229 Conversion Barrel

I believe you'll need a .357/.40 slide. The barrel diameters are different. Or get an aftermarket conversion barrel. I'm not sure if you'll need a different locking block.

9mm to .357

.

The differences between a 357/40 P229 and the original* 9mm version:

The 357/40 slide is heavier and a less powerful recoil spring might be necessary for reliable functioning if a 9mm conversion barrel is used with the slide.

The 357/40 magazine well is wider and although a 9mm magazine will probably lock into place, it will be loose and that might—might—affect reliability.

A factory 9mm P229 barrel will not work in a 357/40 slide because it is smaller in outside diameter. Plus the barrel hood extension of the 9mm barrel is narrower (as I recall).

Putting a complete 9mm P229 slide assembly on a 357/40 frame won’t work because the locking insert is different. As I recall, though, 9mm conversion barrels made to work with 357/40 pistols usually are configured to work with the 357/40 locking insert—but I’m not certain.

I seem to remember that ejectors for 9mm and 357/40 P229s are different, but I’ve never heard of that’s ever being a problem.

I know of no way to convert a 9mm P229 to 357 except to use a complete slide assembly, and then there would/might be problems with the magazines because a 357/40 mag won’t fit into the magwell of the 9mm. That might not be true if your 9mm was a P229-1 because the magazine well is wider and will accept the wider 357/40 mags. But although I don’t know for certain, I suspect that because the P229-1 probably uses the original style 9mm barrels, the locking insert would probably have to be changed, and assuming that would be possible.

*Some of these differences were changed with the 9mm P229-1.

Added: In rereading your post, I probably didn't answer your question directly, but I assume it's more or less clear.

You can go from 40 to 9mm, but not the other way.

I recently picked an older p229 in 40 and I put my 228 mags in it to see about converting it to 9mm. My 9mm mags fit fine in the 229 frame but the 40 mags won't go into the 228 frame.Not sure if the newer 228/M11 has a wider mag shute.

Cazio

This message has been edited. Last edited by: cazio,

Them "few people" are not correct.