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I am going to get a Sig P320, in a compact (Carry). I already have a 9mm in a sig 229. Here is my question, I was going to get a .40 since I have a S&W Shield so I was going to get a P320 in a .40, but I was thinking about .357 sig. Thoughts on .40 and .357? Are they interchangeable? Go.

You can use a 357 sig barrel in a 40 or vice versa, as long as they are a proper fit for the gun. Both won't work from the same barrel though. 357 sig is basically a 40 necked down to a 9mm.