I'm a fan of the .30 caliber battle rifle. For many reasons.
The decision to go with the 5.56mm/.22 caliber was controversial at the time, and remains so. Detailed analysis on soldier marksmanship performance and combat effectiveness hadn't been completed at the time the decision was made and many false assumptions, some of which are repeated in this thread, were made at the time.
The book, "On Killing" by LCOL Dave Grossman, has a very interesting review on actual combat shooting.
While we look back on the days of the Garand winning WW II with fondness and admiration (both deserved) the actual kids in combat were NOT effective with that fine rifle. Several factors were at play: marksmanship was developed shooting at static round bullseyes, not many rounds were spent on marksmanship, and the shooting was all done in slow, untimed fire.
When exposed to the stress of combat: young soldiers were not able to hit their targets. Over 90% of WW II soldiers did NOT hit ANYTHING in combat (from the book). The Army was only beginning to realize this in the early 60s.
The first time a WWII soldier shot at a silhouette - it was an actual human being who was shooting back, and their training had not prepared them at all. Many soldiers found that they simply could not take aim at another human being. This shouldn't surprise us - it's a moral question and we hadn't trained those kids well enough to be able to do their jobs.
The implementation of man-shaped silhouettes, realistic targets, pop-up targets, timed fire, shooting from cover and various positions, all changed the nature of the American Soldier. Vietnam was little better than WWII, or Korea. But by Iraq and Afghanistan, over 90% of our soldiers were returning accurate, lethal fire.
And then we discovered that, at our typical engagement ranges, the M855 round was simply passing right through the target without lethal effects. It was taking 4 rounds, on average, to stop a Taliban fighter, using the 5.56mm M-4 system. Simply terrible performance. We had solved the soldier part of the equation through improvements in training realism and frequency, but discovered that the weapon system was inadequate. Until we had actual soldiers trained well enough to get good hits (as we now do) - the weaknesses in the weapon system was completely masked.
So, while the Army isn't ready to go to an all .30 caliber rifle force, they are spending big $$ (and not making decisions) on improving the 5.56mm round itself by modifying the projectile. Exotic metals, new design, etc. have all been examined. They're well aware that their present rifle is inadequate.