Just got back from some training, but I had a discussion with another LEO who had a problem with his S&W M&P15T. This is the second time in two weeks I've heard of problems with the 15T model. The issue is with the gas block, and the set screws loosening and the gas block shifting, causing the rifle not operate properly. The first sign of a problem is short-stroking due to loss of gas pressure. If you have any M&P15 rifle with a low-profile gas block that is secured with set screws, make sure to regularly check the screws for tightness. Mark them with a permanent marker or a scratch so you can tell if they start to loosen. If this is a problem that persists, the only thing you can do is to pin the gas block with a taper pin. Threadlocker fluid like Rocksett and red Loctite will not reliably hold up to the heat of the gas block.
Just an FYI to anyone with one. Still great rifles, just a small problem to watch out for. It's the main reason why I prefer to pin my gas blocks.