I successfully swapped a Bear Creek barrel, fit the M&P upper like a glove, and a Toolcraft bolt into the M&P bolt carrier without issue. Has feed many mags without issue. My brother has it now, he tried to put a complete Bear Creek upper on it, the profiles matched perfectly but the pin spacing is a few thousand off. He succeeded in getting the front pin in but the upper would not close fully.
So the upper and lower receivers are proprietary as well as the barrel extension and bolt face. Everything else is interchangeable. So if you want to run a 6.5 Creedmore you could but it would not be a separate upper unless you can get your hands on a M&P upper. I tried SM and they will not sell it.
