There's some very fundamental misconceptions that pop up in these discussions.
1) Expense. Changing over completely to a new caliber and weapons might cost millions, but in reality, that's less than two new fighter planes. And we seem to lose that many every year just keeping the pilots trained. It's really a very small line item in the budget compared to say, FUEL. Add up all the diesel, gas, aviation, and bunker fuel used by DOD, a new caliber and weapon is insignificant. That's how far off base these discussions are.
2) "The 5.56 is 'too light.'" Well, it's not surprising in this day and age, with only 1 in 100 citizens volunteering to serve, that the general public missed out on why power levels were reconsidered.
The Defense Department has been studying this since the days they were known as the Department of WAR, and the harsh reality is that the heavier .30 cal weapons aren't well liked or shot by soldiers. Recoil doesn't help. What's even more disappointing are the results of all the armed fire - an enemy soldier is likely to get hit by an unaimed shot as much as aimed fire. We increased the number of bullets being shot by making the gun easier to shoot, having large magazines on them, and giving them less recoil. Now, soldiers shoot them a LOT more, there are more of our bullets in the air, and more enemy soldiers are hit.
AS FIRST INTRODUCED, the M16 shot a 55gr bullet at over 3,100 fps, and if hit, the enemy soldier likely stopped shooting back. That was the goal - getting the other soldier to stop fighting. And where most civilians miss the point is that the other enemy soldier doesn't have to die right there. Just stop fighting.
It's NOT Deer Season, Elmer. He's not likely to run away and hide so you can't tag him and truck him to Hatfield's Processing. He'll be right there to handle during Consolidation of the Objective. Medics can patch him up, MI will debrief him, and MP's will house him.
For every combat soldier, there's 9 support soldiers. They all use the same weapon. What you burden with one, you burden on all. What went wrong with the M4 is that it was meant for support, command, and special units, NOT the combat arms. But, being the competitive guys they are at interservice rivalry, combat arms wanted cool guns, too. Just like everyone wearing a beret, everyone got a M4, and now, the 5.56 was cheated out of 5 1/2" of barrel velocity at 50fps per inch. It lost speed and range.
Bad decision. The Marines didn't do it, the M16A4 in 20" is still standard issue. The Air Force still uses M16's, A1's, and A2's. It's NOT a service wide problem, it's Army Combat Arms specific.
That's the third generalization most get wrong. We don't need a new high power cartridge, we just need to quit playing commando joe and go back to 20" barrels. Which are actually in larger numbers to this day.
FIGHTING INFANTRYMEN carry RIFLES. Those get the job done. When you cut them down, then you might consider a cartridge change. I did, built a 16" 6.8SPC. The Army? Not really needed.