Some comments about the video's linked above:
The "experts" from the Armed Forces Journal" rave about the HK, aren't those the same people that were completely conned by Stan Bulmer from LeMas about their ammo? Temperature sensing bullets and all? They made fools of themselves on that one.
The guy from the Discovery Channel series, I can't think of the name of it, he's easily impressed, by the way, he also fell for a demonstration put on by Stan Bulmer from LeMas.
So what is the HK able to do that the M4 can't, he buried it in sand, the magazine was in place and the cover was closed, M4 would have absolutely no trouble with that. He dunked it in a trashcan partially filled with water, did you notice the Seals in the opening scene in the sea with their M4's? The Seals dunk their M4's all the time, no big deal. What exactly did they show that was different from the M4? The bolt was cold after firing, big deal.... It's cleaner than the M4, true, it is cleaner, but that cleanliness comes with the added complexity of a piston and lots of extra parts that are known to be more prone to failure than the simple system used in the AR's. It's easier to attach and remove the grenade launcher, whoopee...
A lot of the AR guys are real excited by the weapons showing up that are piston operated, but the vast majority of those folks aren't familiar with the very valid reason why the AR was designed to do away with the gas piston.
BTW, M1's and M14's have definitely had their share of jamming problems, and sometimes those jamming problems are caused by the piston actuated gas system malfunctioning.