I'll bow to nobody in my disdain for the AR platform as a battle rifle. Direct gas impingement blows; it is broken by design. I don't care that it eliminates a moving part, the right answer is gas-piston (like the AK) or even better, op-rod. This is the answer to those who claim that it is a perfectly fine rifle as long as it's properly cleaned: (1) sometimes life happens and you have to use a rifle that's in a state other than perfectly clean; and (2) even if it starts out as immaculate, use it long enough, eg in a situation in which the metal bits of the rifle are literally glowing white hot (as described in the NYT article) and you'll find yourself with a filthy rifle you just don't have the time to properly clean. Also, the gas tube can break, rendering the rifle almost entirely useless since the soft metal and lame plastic makes it a relatively poor club or lever.

Granted, the notion of the upper receiver being just another part makes it a nearly infinitely flexible system that supports a plethora of caliber options. This is it's main saving grace as a civilian rifle that is realistically not going to be subjected to the rigors of battle. There are lots of easily-swappable parts available and just about anyone can snap together some pieces that will still shoot quite tight. Of course, the military is standardized on the stock rifle and has stupidly chosen the 5.56mm poodleshooter round which is obviously heavier but has much worse terminal characteristics than the 7.62x51 NATO round. No wonder special forces and other units that can opt out of the rifle do so. Yes, it's partly done because they can but mostly because they really, really need to.

The AK-47 has much, much worse accuracy but has to be considered a better battle rifle because it is good enough in that department and is much sturdier and physically holds up better than the AR platform. The AK line harkens back to a time when battle rifles were battle rifles: the Garand, the Mauser, the Nagant, and Enfield (bolties all except the Garand) were real rifles and shot real ammo. John Moses Browning would be half-impressed and half-horrified at Stoner's creation.

Since this is HN, I'll make an analogy to a poorly designed and written web server. When you're not stressing it, it doesn't matter that you're doing something in O(n^2) when you should be doing it in O(1). But when push comes to shove, no amount of external patches (more bandwidth, more CPU) can disguise the fact that the software sucks.

Oh, and the SKS was designed by Simonov, not Kalashnikov.