5.56x45mm has gotten a bum rap.
What's wrong with it?
The problem is that its problems have been tied up with and confused with the launcher.
In Vietnam the complaint wasn't that the round was failing to stop the bad guys but that the rifle stopped running when you wanted to shoot bad guys. M193 will kill people and has killed a lot of people.
There are many detail improvements to the M16A1 between 1967 and 1974 that eliminate the issues. Of note, the Air Force's M16 did not take part in much of the improvements and did not encounter most of the problems. It's been theorized that the Army's dislike of the M16 caused them to make changes to the detriment of the design of the XM16E1. The M16 and M16A1 are very different rifles in detail.
None of the those problems were carried into the M16A2 and M4.
The M16A2 introduces a new negative to the 5.56x45mm equation. The M855 round. It's not a traditional ball round. It's a steel core surrounded by lead with a copper jacket, and a small air space in the nose. the reason for the steel was because NATO had a requirement that any STANAG rifle round be able to penetrate an M1 steel helmet at 600m. Then there were accuracy requirements.
The M16A2 with M855 passes those tests. Someone seems to have forgotten that it needs to kill people.
Military 5.56 ammo is dependent on the round flying apart on impact, known as fragmenting. M855 drops below the velocity that this reliably occurs before M193 and it also comes apart later on impact than M193. What this means in the real world is that people who live at starvation levels can have a round pass all the way through them before it fragments; making M855 no more effective than .22LR. The .22LR like performance is also true at ranges past the fragmentation velocity drop-off.
A valid criticism of the M4 is that with the 14.5" barrel the initial velocity is that much closer to the drop-off velocity making it even less effective against the kinds of people we are shooting at today.
Something of note in the debate, Israel is staying with 5.56 even though they are on their third rifle design that uses it. I don't see them sticking with a useless round because they are likely to be needing effective fire.
Both M193 and M855 are creatures of treaty. Their bullet construction is constrained.
I can buy hunting ammunition that expands to nearly .45 caliber makes a much scarier wound channel than anything military.
My carbines are not affected by many of the reliability issues the M4 encounters simply because my carbines are all semi-automatic.
Do not hate 5.56. Understand it. Learn its strengths and limitations. Then decide if it can do what you need done. I decided I needed the extra die of damage and 320 yards of 1/2 damage range I get from 6.8. ;)