I think I know what happened, I was shooting over 800 rounds using federal loads about a year ago... I wanted to use up those for my replacement 22 of CCI Tactical loads.. Well cleaning out the lower receiver I noticed small sand like particals, I think maybe powder flakes maybe burn flakes not sure, but it looked and felt like coarse sand... I haven't had that with CCI Tactical and I've put over 1200 rounds of that thru my M&P.. Also add Winchester, I also did about 300 rounds of that load as well with the Federal..