The method shown is helpful with barrels exposed to corrosive priming, but otherwise, why would you want to do this? I would never do this with the lower or the slide. Failure to get the water out of every nook and cranny is just asking for corrosion. While the barrel and slide are hardened with the melonite process, the internals are not.
In addition, with the M&P, GREAT CARE must be taken to not get the "tampon" wet. The "tampon" is the "thing" inside the trigger return spring in the frame that most people don't even realize is there. Without the tampon, the return spring will literally vibrate itself to pieces. Getting the tampon wet will either: (1) cause the tampon to disintegrate, leaving the spring without its vibration protection; or, (2) expose the spring to moisture, causing the spring to rust and break in normal use. Either way, you don't want this to happen when you need the pistol most. Agencies are told to replace this vulnerable part (the return spring with tampon inside) once per year. Similar instructions are seemingly deemed not important for us commercial users, even though the M&P is sold for defensive purposes.
This tampon-in-spring part has been on S&W polymer autos ever since the SIGMA. The original gun magazine torture test of the SIGMA, by a gun magazine with integrity, resulted in then-unexplained breakages occurring after "dunking" the pistol in a barrel of water to cool it. The tampon got wet, "went south" and the spring broke - alarmingly fast. They assumed it was a "fluke," repeated the process and after dunking, the same result. They finally determined the tampon was getting wet, disintegrating and falling out.
Why S&W ignored the problem and continued to include the part is beyond me. But, it is a weak link. While the M&P pistol is a great pistol for civilian use for either the homeowner or the police, this makes its use problematical for military use as military pistols can and have been subjected to "dunking" either on purpose for cleaning or accidentally, if dropped into water, or incidentally, should the pistol get wet crossing a stream, for instance. Thus, I would not be interested in getting an M&P wet, except for the barrel, but only then if it were exposed to corrosive priming or salts, etc. Otherwise, why bother?
