Lethality of 5.56mm

Being a military armorer, I get a lot of discussion about what round is better than the next.  The 9mm vs .45 debate comes up a lot.  I also hear a lot of misconceptions around military ammo, specifically the 5.56x45mm.  My thought process is the best round is the one that you have, that works.  I'm issued a variety of calibers, and have a lot of experience with unusual (for regular troops) military ammo.  In this post I want to go over some of the 5.56mm options and lethality, and what is being done to close those gaps.