Curious about this, as with the dearth of 9mm available, I've been looking at what it would take to convert some of my current guns to .40. The Glocks require the full slide assembly and changing the ejector I'm aware, but what about the M&P? Looking at parts online the ejector and housing are listed as for both, what about barrels and slides? Would you be able to simply drop a .40 barrel and recoil spring in a 9mm slide and go? I suspect it would be more involved than that but want confirmation.