@robertsig: I know its fine. Its great. Its not the best for the role especially when you fire it out of barrels shorter than M16 barrels (20").
*Disclaimer: The following link does favor the 6.8 SPC. But, I'm using it as an example to show the deficiencies of the 5.56x45 round.*
http://www.itstactical.com/warcom/ammunition/military-ammunition-failures-and-solutions/
@Crow Hunter: See link above and consider the following.
1: These rounds are not much heavier... many of them are similar in weight to or lighter than AK-47 rounds. The important thing is that their marginal weight increase (which is very small) yields deadlier internal ballistic performance.
2: These rounds do not recoil a very noticeable amount more than 5.56x45 (many of them akin to AK rounds or less). And, again, they offer better ballistic performance that doesn't rely on high velocities that 5.56x45 never reaches out of a 16", 14.7", or shorter barrel.
3: Some of these rounds offer a flatter trajectory than the 5.56 (6.8 to name one). Others might not be as flat but they are adequately flat out to 300m and they are much more effective when they get there.
4: Penetration is pointless if it overpenetrates before yawing/fragmenting. These other rounds don't have problems penetrating and have tangible affects on soft targets.
5: Well, if one of these rounds is adopted it will be available... so this is a moot point. We are talking about "better" not "most common".
6: If one of these rounds is adopted then it will also cost very little... so this is a moot point. Again, we are talking about "better" not "cheapest"
7: Lethality. It is lethal, but a .22lr is lethal, too (don't get me wrong... I'm not equating them). I'm just saying that 5.56x45 isn't as deadly as these other rounds and they do not suffer as many of the downsides of using a full house caliber like a .308 (which you claimed that they do judging by your list). They aren't that heavy, they don't recoil that much more, they penetrate better, they do not suffer a loss in magazine capacity (generally), etc. They aren't 7.62 NATO rounds even though some of them are .308" in diameter.