I have a FS M&P 357SIG that I matched with a 40S&W barrel. I have a M&P9c that I have matched with a 40S&W barrel. I also have a FS M&P45. I have carried a 45acp professionally and CHL (Colt 1911 or Officer's ACP) for 30 years. I think that the 45 is an outstanding bullet diameter. The 40 is not far behind. The 357- SIG or magnum is a great shocker with 124/125gr bullets. I carried a magnum before I switched to the 45acp. I would feel well armed with any of the three, so I won't try to sway you on any one of them.
Barrel conversions are straightforward between the M&P 40/357 full size pistols. Simply swap out the barrels and use the same magazines.
Going from 9mm to 40 requires some barrel fitting (At least on my M&Pc). I had to shave the chamber sides at the rear of the chamber to facilitate barrel/slide fit. I also had to slightly tweak the extractor and polish a sharp corner on the breach face to get reliable feeding. Also need the dedicated 9mm mags.
-If you go with the 357SIG, get ready to reload. Here's some tips: Starline has great brass and no shipping charges. Do NOT try to use Berry's or Ranier plated bullets. Ramshot Silhouette powder will give max velocities with no pressure signs. Plus, Silhouette powder will help reduce your muzzle blast and almost eliminate muzzle flash.

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