This was going to be my next guess.

Also in terms of lower receivers. It makes u wonder if the M&P is a little bit more flex after the round goes off. If this is the case, the follow ups would be a little more off. I mostly have experience this with the AK varients, so i've never looked into it closely enough in the AR platform. But we all know that different companies make different products, some being a more solid product. It would make sense, atleast IMO.