I think we concur on a whole lot of this discussion. That’s a good thing!
I guess I missed the 6.8x39 idea-I never saw much utility in the AK weapon overall. What I do know is that a lot of 7.62x39 rounds get thrown at us (general direction anyway, not too much aimed fire in my experience (a very good thing!), while we paid some more attention to putting the dot on the BG and, well, you know.
Ref. the concept of intermediate caliber combinations-they all have fallen by the wayside, excepting the 556 (speaking of western counties anyway). Don’t hold your breath for a new service cartridge!
I think the guts of the problem is we are still using centerfire cartridge technology that is well over 125+ years old. Where are teh JMBs of teh 21st century? (oh, right, we pump billions into non-lethal etc, when the bottom line is BGs need to be killed)
Our mid-bore cannons (25mm Bushmaster etc) operate at a whole lot higher pressures (~125k psi), they hold together and are very potent, yet our small arms are still limited to 60k psi....
I have a Ruger M77 270 win that I double charged with a reduced load of pistol powder (short range practice, and it held together-case head expanded about .050”, primer was never found and the bolt had to be beaten open with a leather hammer. I was too old to be turned over my Dad’s knee (I was about 15) but I got a very stern lecture about QC in the reloading process! I always do basic brass checks for case head expansion with my reloading endeavors.
As the late Phil Sharpe once wrote, and the Angels sing....