Barrel swap in Smith & Wesson M&P40

Question
I'm getting a Smith & Wesson M&P40 and I want to get 357 SIG and 9mm barrels so I can have 3 in 1, but I want to know if there is going to be problems with shooting the higher SAAMI pressures of the 357 SIG cartridge in the M&P40? Or are they made about the same? Would I be better off getting the M&P357 instead and getting a 40S&W barrel? Thanks for any help you can give me!

Answer
While the M&Ps probably are the identical guns, made specific to calibers, I think it is always a good idea to get the gun that is rated for the highest pressures and do conversions "backwards." So, yes, if I was doing what you are doing, I would get the gun in .357 SIG and a .40 conversion. Be aware that for the 9mm the breachface will be too wide and the ejector will be in the wrong place (.357 and .40 have identical sized case heads). This doesn't mean you shouldn't do it, or that it wouldn't work, but I would not recommend the 9mm conversion for anything but fun and games. I would not use it for a self-defense gun, or anything that I would trust my life to.