I noticed MidwayUSA had S&W 9mm barrels in stock last night and I placed an order for two of them because I have two friends with M&P40s. Unfortunately one of them didn't like my nice gesture and surprise and he decided he was gonna buy an entire separate 9mm instead... so I called Midway to edit my order but they already shipped it. Potterfield has some damn good customer service.
Anyway, I will have an extra M&P 4.25" 9mm barrel. If anyone is interested in purchasing it from me, it is on the market. If you have a 4.25" M&P40, the barrel will fit, and it will function. I've tested 3 separate M&P40s with 9mm barrels and all ran 100%.