XD-Ro,
As previously posted, yes, the .40 magazines work for the 357 Sig XD. Half my magazines for my .357 XD are labeled as .40's and half are 357 Sig.
ampleworks,
Actually, the 357 Sig isn't the .40 S&W necked down, even though it looks like it. The 357 Sig case is thicker in both the web and at the base, just above the web, to withstand the higher pressures of the 357 Sig round. If you simply neck down .40 S&W brass, the case comes out considerably shorter than the Sig round, leaving a very short neck and a case that is dangerously overstressed when loaded to full power 357 Sig specifications.
There is some speculation that the 357 Sig was actually a derivative of the 9x25 Dillon, which is the 10mm case necked down to 9mm. There was some interaction between Dillon and Sig prior to the introduction of the 357 Sig cartridge, and a Sig representative took some samples of the 9x25 back to Sig about a year before the round was announced. There is the difference of the primer pocket sizes between the 357 Sig and 9x25, but basically, the 357 Sig round more closely resembles the 10mm case than it does the .40 S&W, other than the different primers used. This information was given to me by a former Dillon employee who worked on the 9x25 project.
XD-Ro,
If you really need more than 12 rounds for the 357 Sig (or for that matter, the .40 S&W) in the XD, there is a way to do it. As an experiment, I used a Pearce +2 grip extension and the spring and follower from a LEO marked .40 magazine and I was able to get 15 rounds of 357 Sig ammunition into it. The springs and followers are different in the LEO magazines and the 10 round post ban magazines. With the post ban follower and spring, I was only able to put 2 more rounds into the magazine.
As a side note on springs, the 10 round 9mm Compact magazine spring is about five coils longer than the full size 10 round 9mm magazine spring. Evidently, this was one of the means that was used to restrict the compact magazine to 10 rounds. They simply made the "spring stack" longer, so the follower couldn't go deeper into the magazine.
Hope this helps.
Fred