Yes it has. And not for the good. Let me respond to a whole bunch of posts instead of one-by-one replies.
The 5.56 is an anemic round. It's adoption has far more to do with what DSK wrote of than any kind of ballistic superiority.
I am in a military reserve component. I also work security for the feds full time. Any armed person, for the most part, cannot shoot as well today as in generations past because of the disapperance of the firearm as a staple in American life. For the last twenty years or so, kids have been at the helm of a video game control box instead of shooting birds, squirrels, rabbits, etc.... This has had a horrible impact on the shooting abilities of those who are introduced to firearms in their adult lives. I started shooting at about age 8. Two days ago I went to qualify with my trusty old S&W model 10. I shot a 290 out of 300. A year ago I shot a 285 out of 300 with the Beretta M9 at my Guard unit. Many men and women (civilian and Guard) cannot meet the minimum passing score of 180. In the military, they are offered many attempts. After many unsuccessful attempts, their records are forged to show a passing score (same with Physical Fitness tests, by the way). I've seen it many times. In my security job, those folks who cannot qualify are now starting to be fired. In the very recent past, their records were forged, too. So, as DSK wrote, the common thinking is to "pray and spray." I do not agree with this.
It is not too different from what happened at the FBI around 1990 or so. They wisely chose the 10mm as their primary duty cartridge. The S&W 1006 was going to be their duty weapon. Too many men, and especially women, could not handle the full power of the 10mm. They downloaded it. Somebody at Smith got the brilliant idea of shortening the case to accomodate the lighter charge. Voila! The .40 S&W. Shortening the case and reducing the energy also meant the size of the gun could be recuded from a .45-sized frame to a 9mm-sized frame. Have you noticed the FBI HRT and many Armed Forces special ops are slowly returning to the .45?
As for penetration, the 7.62 is superior to the 5.56. Somebody here was comparing 5.56 armor piercing to 7.62 ball. Pulllleeeeeeasse. Let's compare apples to apples.
Ballistic coefficient has little to do with penetration. BC measures aerodynamic efficiency. That's about it. If two bullets of different weights leave two barrels, with the same muzzle velocity, at the same time, both with the same BC, it does not mean they will impact their targets at the same time. This is where the weight comes in. A heavier bullet will impact first because of momentum. A heavier bullet retains more energy. It does not become evident until you get out to 600 or 700 yds and beyond. This is where light bullets "crap out" and heavy bullets keep on hummin' along.
In my opinion only, the 5.56 has been adopted to accomodate a new generation of video-gaming, mall-skateboarding soldiers who are inexperienced with arms. Give them a lot of bullets and a gun that recoils lightly, and hope like hell they hit something, of course not getting some hot brass tossed back and caught in their nose rings.

Rob
[This message has been edited by In service to His Majesty (edited 10-27-2001).]