I've recently bought a M&p15 Sport II. It seems to be nicely made and I haven't had issues using that rifle.
Today I stripped the BCG and noticed that the Cam Path of the BCG is machined differently than Mil-Spec Carriers.
There is a notch sticking out on the Cam Path where it should be completely smooth. However, after taking a close look, I noticed that the notch wont interfere with the Cam Pin Clearance.
Otherwise I haven't had any issues with the Carrier. It runs very smoothly.
Do you also have this notch on your Carriers? I'm worried if it's out of spec or normal on M&P rifles.
That cam-pin cutout isn't right. The cut out looks like it was over cut or hogged out with a dremel. I would call S&W Customer Service first thing, explain the issue and offer to send the photos. See if they will just send you a replacement Bolt Carrier as that is a user serviceable item.