I have full size S&W M&P .40, then I bought a 357 barrel for it, that's all you have to do is swap the barrel and you are good to go, the .40 mags that come with it are designed for .357 as well, so a simple barrel change and you got another caliber. You can go to 9mm with it too, but gotta get 9mm mags. Love the versatility of it and how it shoots, just my 2 cents. My CCW carry is also a .40, Walther PPS, and I love it as well. Like others have said, you just need to find what fits you and is comfortable to you, I personally am a .40 fan, but it is also my duty caliber as well.