Interesting how the quest for the ideal main battle rifle cartridge has led back to conclusions that were made - more or less - around 1900. Well, why not? Most current service pistols are using the modern-as-tomorrow (if tomorrow is 116 years ago) 9mm Parabellum.
I think the 30-06 and 8mm rifle cartridges were pretty good. I am not a big fan of 9mm and prefer 45 ACP. In expanding rounds, modern 9mm are getting better.
Ballistic coefficient has favored 6.5 bullets for a while. My garand weighs in at about nine pounds or more. Clip capacity 8. 6.5 Grendel about 7 pounds. 20 round 5.56 clip holds 16 with the right follower. Muzzle velocity is about the same. Bullet weight .30 cal. 168 grain 6.5 123 grain. Honestly I don t know what’s the best. Going back won’t work and changing all 5.56 rifle barrels and bolts and clips seems prohibitive. Then the supply of ammo to any new round. This whole issue is a bear that I am glad I don’t have to wrestle with.