So today I start with a disclaimer that I am only passing information along. I am not a gunsmith and so I cannot verify this conversion will work reliably in every caliber with the exception of the caliber of the original gun.
Now for the fun. I’ve been thinking about a Smith & Wesson M&P .40 to add to the collection so I would have a pistol of that caliber. The M&P gets great reviews and people who have them seem to love them. I really don’t need another poly gun, but if S&W puts out their instructor list and it is on it at a decent price I might decide to pull the trigger on a purchase (pun intended).
Yesterday I was perusing a forum and the discussion was about converting the M&P .40 to .357SIG. As I understanding the .357SIG is essentially a 9mm in a 40 casing made for semi-automatic pistols. An oddity of the M&P is the only appreciable difference between the .40 and the .357SIG is the barrel. The magazines are the same, or at least will function properly without any worries so one could purchase the .40 and then get a .357SIG barrel and complete the conversion in a matter of minutes. You may be wondering why is that a good idea and the answer is the availability and cost of ammunition. If one caliber was becoming difficult to obtain or expensive you could purchase the other. I see it as pure genius!
In my best infomercial voice, “BUT WAIT THERE’S MORE.” Another quirk/perk is the 9mm barrel also fits and operates in the .40. The only drawback is the magazines will not so you will have to purchase 9mm magazines. So for an additional $200 and change you can have the potential for three different calibers in the same frame and slide. That really is amazing.
The key if you attempt this will be to know what barrel is in the firearm and ensure the ammunition is compatible. The slide will still state M&P.40, but the top of the barrel above the chamber will be key to identifying the exact caliber in the gun.
Back to disclaimer, I will go ahead and tell you not to attempt this conversion. Just don’t do it. Stay away from it. Run, run away! (There, that should make it easier for me to purchase the barrels and magazines.)