Most of this is fanboy stuff anyway (Oops, gonna get flames on that one).
By Fanboy I generally mean factory support. If you shoot mostly factory ammo, then off-the-shelf ammo matters.
Advertising budgets usually determine popularity.
If you reload, then off-the-shelf components matter.
If you look at most AR-15 compatible rounds, they generally follow the .223 type case, necked up or down, the 6.8 SPC type case, necked up or down, or the Grendel case, necked up or down. Sometimes the body length or overall case length is shortened or lengthened, but those case styles cover most of what we find. Sometimes the heritage of the case shape matters, sometimes it doesn't.
The bullet makers make or break a cartridge. Powders that burn in the first 10 inches, to powders that are still burning out the end of a 20" can optimize most of these for a limited range of performance. Twist rates work their way into the evaluations, like the 224V being better than the 22N because of the 7 twist vs the 8 twist initially available.
With a finely divided diameter range available, like this;
5.56mm 5.7mm .224"
6mm 6.2mm .244"
25-45 6.5mm .257"
6.5mm 6.7mm .264"
6.8mm 7.0mm .277"
7mm 7.2mm .284"
7.62mm 7.8mm .308"-.310"
Why can't bullets, cases, charges, barrel lengths and twists
be designed for specific applications in most of those?
Which one is the BEST?