Just a little information. All M&P pistols in 40S&W will allow the swap to the 9mm barrel. I doubt it was engineered by S&W to be a drop in barrel swap, but more likely just done to make manufacturing easier and less expensive. The 9mm and 40 basically use the same slide and barrel dimensions. S&W states it doesn't work and should not be tried. However there are thousands of M&P owners that have used this barrel swap, and I have never read of a single issue with the barrel swap. People often ask which is better the 9mm or 40? If you own a M&P, the answer should be to buy the 40, and then purchase the 9mm barrel, and you have two calibers in one gun. The swap is size related, so if you have a full size, then a full size barrel, etc. I own the 40 Shield and was able to purchase a stock Shield 9mm barrel ($70). I can be at the range shooting 40 ammo, pull the slide, pull the 40 barrel and put in the 9mm barrel, replace the slide and start shooting 9mm ammo. On the M&P models, the extractors, ejectors, and recoil springs are the same between the 9mm and 40s. I also found my 40 mags allow perfect feeding of the 9mm cartridge, but others have had feeding issues on the last round in the mag, when using the 40 mags to feed 9mm ammo. It is a feed lip width issue with some of the mags. I did purchase a few 9mm mags, which also work perfectly. At 15 yards the groupings are identical in size, however I found the 9mm to impact 2 inches right and 2 inches lower than the 40's impact point, with the same point of aim.
This only works 40 to 9mm, and does not work 9mm to 40, as the upper rear barrel tang width is slightly wider on the 40 barrel, and will not fit into the narrower tang slot in the 9mm slide. I use my Shield as a 40 for SD carry, but shoot a lot of 9mm ammo, at the range. Cheaper and less recoil/flip. The major issue is finding the 9mm barrels, as they are scarce, and in big demand. I don't know if any other brands allow this easy barrel swap, but it works great for S&W M&P 40 owners.
Bob
This only works 40 to 9mm, and does not work 9mm to 40, as the upper rear barrel tang width is slightly wider on the 40 barrel, and will not fit into the narrower tang slot in the 9mm slide. I use my Shield as a 40 for SD carry, but shoot a lot of 9mm ammo, at the range. Cheaper and less recoil/flip. The major issue is finding the 9mm barrels, as they are scarce, and in big demand. I don't know if any other brands allow this easy barrel swap, but it works great for S&W M&P 40 owners.
Bob