Some thoughts:
I don't know the legal status of the shotgun but every command I've ever heard of in the navy uses these extensively, even (especially) forward-deployed ones. It is an extremely common weapon.
M9 problems: everyone complains about its lack of stopping power. I'd like to see statistics. Fact is,
very few of the people who are issued the M9 are ever going to use it in anger. How many of these soldiers who complained about its lack of firepower actually shot someone with one? (And why weren't they using their long gun instead?) As for the magazines...what are they doing to them? I'm guessing that the sand can prevent the rounds from rolling properly through the top of the magazine, but I've never witnessed an M9 magazine fail like this. The potmetal baseplates popping off are another matter. There is a scattering of tritium-equipped M11 pistols here and there; I'm not sure of how many the army got hold of or where they were sent, but I see them from time to time.
By the way, I'm not defending the M9. I don't like it either. It's not a bad pistol as pistols go but we could've done a lot better. That's why I don't like it.
M4's lack of range: it was designed to be a replacement for submachineguns. One more time: it was designed to replace submachineguns. It was thought to be a better weapon in this regard than the M3. (I agree.) I don't know what the army's line of thinking was with regards to making it a frontline weapon. It was not intended to be used at 500 yards. (It was not intended to be used at 300 yards, for that matter.) I just don't get it.
M203 "buckshot" round: this is a low-velocity weapon using the odd high-low pressure system developed by the Germans in WWII. It was not intended (and is not able) to launch things quickly. A buckshot round was tested in Vietnam. It did not work well due to its low velocity. A flechette loading was also tested. That didn't work well either. One could change the pressure system to a standard one but the recoil impulse would likely kill the firer - hence, the strange method of operation and its use for launching grenades.
How about a beehive round in 120mm instead?
Gerber multitool: don't leave home without it. Even us squids know that.
I won't comment on the other stuff - the comments regarding the things I have direct experience with, I agree with. For what it's worth, though, if you spend your day standing in icewater, and you're independantly wealthy, Thorlo Combat Socks are the way to go. (At ~$9 per pair, just buy one set and wear them all week.)
Edit: forgot to mention this. Regarding the safety on the M16, dimpled primers, etc...the M16's firing pin is not under any sort of spring pressure. It floats freely in its channel. If you chamber a round it will dimple the primer a bit - nothing unusual or frightening about this if you're using military ammo. (Before anyone complains about how unsafe this is, remember that the M14, M1, and Kalashnikov series of rifles - to name a few - all use the same system.) It's theoretically possible to get a slamfire upon chambering a round...but I've never seen it happen. The chances go up if you're dropping the bolt on a round already in the chamber.
If I had to guess, I'd say that the latter happened, or the soldier simply had his finger on the trigger. In any event it's always a good idea to keep your muzzle in a safe direction while chambering a round, regardless of the weapon's design.