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Terry, in order for that to function you would need a new slide and new magazines at the minimum, I am not sure if a new frame would be necessary. The bolt face intended for .40 S&W will not hold a 9mm case securely due to the differences in rim diameter.

Terry, yes you can. This has been done in this Smith and Wesson thread (just use a 9mm Magazine) : http://smith-wessonforum.com/smith-wesson-m-p-pistols/170963-test-9mm-factory-barrel-m-p-40-a.html

I've seen people swap a factory M&P 9 barrel into the M&P 40 slide and it seemed to work, but the manufacturer does not recommend doing this!

Yes, 9 /40 utilize the same slide & frame. S&W sells replacment barrels directly, i can personally vouch for storm lake.

Terry, Absolutely you can put a 9mm "conversion" barrel in a 40S&W M&P. I've done it. Can not use stock 9mm barrel. Must be the 40 to 9 conversion barrel because of outside diameter of barrel. Will need 9mm magazine, no other changes needed.

Yes, but you'll need to buy a conversion barrel. Factory barrels will just cause major issues eventually due to the smaller factory barrel diameter in a .40 S&W slide, if it would ever work properly. You will also need the 9mm magazines.