My P2000SK in .357 SIG came with two 9-shot mags with flat floorplates. I would like to get a few USP Compact 12-shot mags for it as well as extended floorplates for the 9-shot mags. I have been having trouble finding USP Compact mags in .357 SIG that are actually in-stock. My first question is: even if it is only labeled as one, are the .357 SIG and .40 S&W mags the same? They certainly should be, since they are for just about every other manufacturer. If not, then does anyone know of any places that actually have USP Compact 12-shot mags in .357 SIG available right now? Also, regardless of what websites that sell them claim, are the P2000SK,P2000 and USP Compact floorplates different? Are the floorplates different for different calibers of the same model; because the flat floorplates on my .357 SIG mags say ".40/9mm". I ask all of these questions just to know what I can and can't use with my gun and the mags that I already have, since websites usually specify only a certain caliber; but I have reason to believe that some mags and floorplates are interchangeable. Thank you in advance for any advice.