Yeah, exactly. So the answer is no - nothing in that general family of .45-50 cal, fit-in-.223-magwell, short-range magnum-buttstomper rounds will have off the shelf ammo at Academy, if that's what you mean. But I think they do have their place, as a heavy-barrier penetrator, or maybe a walking-hunting pig gun - very limited place however, other than cool factor. Of those, I like the .50 Beo the best of that general category of round.
I can only agree with this if we are sticking to the black rifle variants. Honestly, I think they were solutions to non problems, or solutions to problems that could have been fixed by going to a more suitable platform.
Out of the .45-.50 "rifle/non pistol caliber" AR casings, I gotta go with the .458. Preference based off a mixture of the ballistics and backstory.
For reference, I have not shot the .50B, the .450B or the .458S. All statements made off the "ammo availability, ballistics, intended platform" conjecture.
6.5G is still hard to find, and would never be one of my options.