>the relationship between AR15 ammo types
Except it doesn't. It just mentions 5.56. The AR-15 is a platform that has been converted something like fifty different ammo types.
.223wyld subforms, like your article discusses,
7.62x39, as a competitor to 300Blk which, like .40short-and-weak shouldn't really exist but does
6.8SPC and its same-brass offshoot .224 valkyrie
6.5 grendel, envisioned for longer range expectations,
.458 socom / 50 beowolf for massive bullet weights
all the pistol calibers: 9mm, 10mm, 5.7mm, .22lr
And that's before getting into popular wildcats like .277 wolverine, .45 v-rap (or the full 45 RAPTOR if you include ar-10 builds) .338 spectre, .400 hamr, probably a lot more.
I don't even notice articles like the one you posted because they consider only a couple popular loadings of 556, without even looking into the full breadth of what can be done by handloading. A good article on bullet selection would be appreciated – a plinking round, a varmint round, a home defense round.
There's a *lot* more to the AR-15 than .223 remington.