.....I thought relying on the case mouth for headspace in a rifle cartridge was considered a bad idea/design, though I cannot say why, seems like I read it somewhere?
I'm sure you did read that somewhere. It's been in print for generations, and not entirely without valid reason.
Here are some points to consider, (and are general, not AR-15 specific)
Rifle rounds are generally longer and often use heavier bullets in the same calibers as pistol rounds. Gun designs differ, and many things common to older designs don't apply quite the same way to newer ones.
IF the gun design feeds the case rim up under the extractor that's one thing. If it doesn't, and relies on the pushing the case into the chamber and then the extractor snapping over the rim because the case is stopped by its headspace point, that is a different matter.
Long and heavy (compared to pistol rounds) rifle rounds get a greater "running start" going into the chamber, and they have greater inertia to keep going until stopped. IF what stops them is the case mouth headspace ledge in the chamber, consider that rifle rounds hit that ledge "harder" than pistol rounds and sometimes its hard enough for the straight rifle case to push past the "stop point", which is never a good thing.
With gun designs that are variations of controlled round feeding, this is rarely an issue, and indeed, depending on details of individual guns and ammo clearances, cases may never actually touch the chamber headspace ledge, and effectively headspace off the extractor.
Specific to the AR design, its not a controlled round feed system. Rounds are pushed ahead of the bolt face until they get stopped in the chamber, and then the bolt finishes it final movement ensuring extractor engagement.
System works just fine with bottle necked cases, SHOULD work ok with straight wall rifle rounds too, but sometimes doesn't work as well as it should.
In the old days, designers didn't have a century + of experience about what works best, and what doesn't to look back on when designing new rounds or gun systems.
For example, every pistol round Browning designed before the 9mm Luger was made used a "semi rim" for headspacing. Browning wasn't convinced headspacing on the case mouth was reliable enough. The 9mm Luger proved it was, and the rounds Browning designed after that were true rimless cases.
The first true magnum rifle rounds (H&H) while bottlenecked didn't have very much in the way of large shoulders and so H&H added the case belt, for reliability, and that was in bolt action rifles, not semi autos.
The .400 Whelen (essentially a straight .30-0t6 case) got a notorious reputation for not headspacing reliably on the tiny, narrow bit of case mouth it had.
Additionally, large bore straight rifle cases with their heavy slugs requre a degree of crimp to function well in repeaters. Even taper crimping squeeses the case mouth against the bullet some, and this also reduces the "shelf area" of the case mouth slightly.
Its a known issue, and can be overcome or its likelihood reduced, when enough thought is put into the design, but its not simple, ABSENT something other than the case mouth to headspace on.
