The invincible metal bra...

Pistol whipping hurts more than shooting someone in the face.
You'd think that unloading a clip of ammo in someone's face at point blank range would deal more damage than whacking them upside the head with your gun, but you'd be wrong. Just try playing a round of Rumble Pit in Halo 2. You'll be surprised at all the people you can kill with the butt of your rifle. That'll teach those fools to use their guns to shoot.

Explosive barrels are abundant in high security areas.
If you were in charge of a high security military facility, would you litter every room with tons of highly explosive barrels? Blowing up bad guys by popping the infamous red exploding barrel is a ton of fun, but that still doesn't explain why they're everywhere and why guards with shotguns like to stand right next to them.