now I will disagree to an extent. I have two ar15 based models that’s shoot a 30 caliber load. Only difference is as you said the barrel bore. I can put my 300 black out and my 30 rem ar uppers on my 223 lower and in fact use the same magazine. And there are a few others that will do the same. 450 bushmaster comes to mind. These are all consider a high caliber as I have been taught that work on a standard ar15.
I know the far left doesn’t know that but in fact they are technically correct with the exception of the 223/556
I always thought High Calibre referred to any rifle round larger than .45, and tended to be used on very large or dangerous animals for hunting.
I always thought of high caliber as .50 BMG and up…
the term was coined long before the .50BMG stepped out the military realm - and if you consider .45 as the threshold in US army talk than it still follows that defination.
It flip flopped for a time between straight walled versus necked (and pistol versus rifle) by when some of the Big Game Cartridges were just huge pistol it went back to anything bigger than .45.