[carguychris]

A few thoughts...

• All S&Ws are test-fired at the factory. A small but significant minority are put through a more rigorous test-fire routine; accounts vary re: how many rounds are involved, but most accounts seem to agree that it's at least several dozen.
• AFAIK S&W categorically does not clean guns after the test-fire routine.
• Regarding the rail wear, I don't know what you consider "pretty worn", but I have never seen a new M&P that did not have some of the black finish worn off of the frame rails (or frame tabs, really) at the slide contact points. I suspect that S&W workers manually run the slides back and forth several dozen times to eliminate sticky spots that could make the gun jam right out of the box. (I know that some other manufacturers do this.)
• Always, always, always research how many mags a gun is supposed to come with, prior to buying. Mags are easily misplaced, but gun store owners realize this, and should be expected to throw in a free spare if the pre-purchase inspection comes up short. BTW I am not aware of any M&Ps that come from the factory with 3 mags, other than the "Range & Carry Kits" that also come with a holster and an UpLula mag loader.
• Did the gun come wrapped in brown wax paper with little blue S&W logos on it? New M&Ps are shipped slathered in gooey and somewhat smelly preservative oil. Missing paper and/or oil would be suspicious.