6.5x50 is a lot closer to a full power rifle round than they are looking for. Also, the 5.56 sucks against modern opponent body armor.
Read more here: https://nationalinterest.org/blog/buzz/what-we-know-about-armys-new-bullets-68052
With the right bullets and a long enough barrel, the 5.56 is excellent at penetration.
“556 sucks against modern armor”
Well, that’s a given, seeing that level 3 defeats everything up to the and including the 762x51 M80 ball, and level four stops 762x63 ( 30/06) AP.
So, basically unless you want to go to a greater than 30/06 round for general use, NOTHING small arms wise defeats modern Level 3 or 4 hard armor. Been that way since around 1928.
On the other hand, a man wearing a modern body armor ensemble (L3A soft supplemented with L3 or 4 plates, whether steel or ceramic/HDPE) is a formidable opponent. Thankfully, we do not encounter such kitted threats; however, in the event of a modern opponent, indeed, men will still be wounded and killed by small arms, but precision will replace suppressive fires.
The 6.5x50 is about what I like. It just needs to be in a smaller bodied case, with modern design to not require complex feed systems/magazines ( less body taper) but capable of pushing that 6.5 120-130 grain hybrid bullet great soft tissue and better armor/material penetration than a simple ball round to around 2900 fps from a mid sized rifle.