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Doing a little bit of looking around on S&W's website, some of the M&P rifles day just 5.56 under technical specs, and some 5.56/.223. The barrel says 5.56, so that leads me to believe that the upper would be for 5.56, but I've been known to be wrong before, and honestly don't really know as much about these rifles as some other AR variants.
What does the rollmark on your lower receiver say as far as caliber?
That is the kind of damage I would expect from ammo with too much pressure. Or possibly after a squib round with the bullet stuck in the barrel (which you say you checked for). You probably won't be able to get the rifle apart at all, because the bolt is probably partially in the buffer tube's area, since it isn't seated all the way forward.
I would contact both S&W and Remington, and see what they can do to make you a satisfied customer.
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Fortes Fortuna Juvat 
We're mules lad. Mules that kill. 
TJR
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