Truthfully, how much do generals really know about bullet wound characteristics, penetrating power, kill/disable ability, etc. There are too many retired generals working for arms manufacturers to leave the choice up to them.
In many categories the M4 falls short of other rifles. For one, the M4 has such close fitting parts and tight tolerances that dirt will foul it much easier than their competitors. The “perceived lack of stopping power” is real. At distances over 100 yards, the bullet lacks the velocity to yaw and cause a larger, more debilitating wound than other rounds. Many, many soldiers tell of shooting an enemy combatant and watching them run off as the bullet zips right through them causing only a tiny hole and little damage.
“Many, many soldiers tell of shooting an enemy combatant and watching them run off as the bullet zips right through them causing only a tiny hole and little damage.”
At 100 yards the standard 5.56 round has more energy than a .45 ACP does at the muzzle.
Granted it's not delivering all of that energy into the target, but no Mil-Spec round ever will.