Why is XXX an accurate Cartridge?

When writing about the "new" 300 AAC Blackout cartridge (which is said to be plenty accurate) - I began to wonder about accuracy and just what factors (if any ) - can be inbuilt into a design to make it more accurate.

And I was thinking - what, if any, - useful stuff might appear - if I compared the 300BLK directly with the 'Russian' 7.62X39mm - and the round that was specifically used in centre-fire 'Bench-Rest' competition 6MM PPC (or/and the .30" BR)

- Well you could have "knocked-me-down-with-a-feather" when I read on Wikipedia the following:

" .. around 1975, when the wildcat 6MM PPC, based on a modified .220 Russian case (which is itself  based on a boxer primed derivative of the military 7.62x39mm), took over as the most accurate cartridge."

So, maybe there is some link out-there other than the one in my meandering thought! - or perhaps way-back I might have read something about this and it's quietly stored in some part of the mushy old brain.

 

And then we have:

3x 300 Blackouts(7.62x35mm), - a .223"(5.56x45mm), and a "Russian" 7.62x39mm.

And just for comparison - a .30 Carbine round (that is 7.62x33MM):

Or - just maybe - there's NO linkage at all really other than the reputed 'ballistic sweet spot' that is supposed to hover around 7MM plus or minus half a mm or so.?? - Is it that these are all just medium (intermediate) sized cases stuffed with a medium sized projectile ?

 - Maybe accuracy all comes down to precision in the production of the cartridge components, the metering of the powder charges, and the tight, optimum sizing of chambers, bores, and rifling - regardless of case and bullet size. Of course I do know about the "boat-tail spritzer" design factors as used in very long range shooting - so why don't we strive to introduce such characteristics into every projectile as far as practicable ? - Because OTHER factors take precedence - such as having a large meplat, or an expanding/soft nose..

- Has anyone ever recently done a properly scientific analysis of this issue?

Anyone got a useful comment to add?
Marty K