Not to bust your balls, but you guys put 31 rounds into a cow and didn't land a kill shot? I have seen countless cows put down with a single .22 round to the head.
Busting the wrong balls, these rounds were all fired directly into the head from a range of less than 5 feet. Cow had charged one of our guys and tossed him, then just fallen over and was laying down frothing at the mouth, the 5.56 stuff was just ricocheting, or penetrating into the bone and being stopped. The 9mm that was fired didn't appear to penetrate, but there was a lot of blood by that point, so it could have penetrated without leaving a wound visible on the surface.
Then again, maybe it was just the animals in Iraq, as a dog that came at us took 10 rounds from the bradley's co-ax (7.62mm) to go down, and it charged the tank after the first round impacted. Another dog took a bullet to the head, from every person in our patrol as they walked by in our attempts to put it out of it's misery, and it was still snapping teeth and growling after 30+ 5.56 rounds. LT's 9mm stopped it.
Insurgent running across a field took over 100 rounds of 5.56 to the chest... we found his bloody shirt at a doctors office the next day, and it looked like it got run over by a lawnmower, but the guy managed to get to the doctors office after being shot to hell by 5.56.
I could continue listing personal experiences with the failure of the 5.56mm round to stop the target, but anyone who loves it won't listen and anyone who has actually used it to try and stop a bad guy doesn't need to hear it.
You say this but some of us know better. I'm guessing you're "one of those guys" that has never owned a polymer framed handgun?
The only way to kill a Glock/XD/etc really with physics is to shoot it. That's the only way I've seen a polymer gun fail in such a fashion. If bullets are hitting your sidearm, you have bigger problems, more than likely, then the kind of gun you are carrying.
-Mike
Never owned a polymer framed gun, though I am eyeballing an xdm right now. I don't say that I wouldn't trust a polymer gun because I have never owned one, I say it because we had STEEL items breaking due to the stresses we put on them. With all my gear on, I weigh over 330 pounds(I weigh 155 normally), and I have watched rucksack frames snap, knives break, barrels get bent, and sights snap off. A polymer gun would "probably" not hold up to the stresses it was put under in the line of duty.
Another point on the "a g21 would be better" point: the average army soldier gets the opportunity to fire about 100 rounds a year through their primary weapon(typically an m16/m4 variant). The guys using an m9 get to put around 50 rounds through it a year. If they are not in the middle of a range fire, do you think they can draw their weapon out of the arms room to practice drawing and firing? I wouldn't trust most of them to have trigger discipline enough to draw a glock without NDing the ground, themselves, oor their buddies 3/5 times; at least the 1911 has a safety on it that will keep them from firing randomly when they draw the weapon by the trigger. Hell, when I deployed I didn't even know we HAD m9's until they issued me one!