I haven’t heard of .500 blackout before, and google gives conflicting info on whether it’s “necked down .338 lapua magnum” or “like .510 whisper”

polar bears have historically been felled with “panicked shooting with ar-15”, and the “standard recommendation” seems to be “magnum rifle round”

.44 magnum is barely on par with an intermediate rifle round like 5.56 against large game. And that’s before considering the massively lower felt recoil or the fact that a rifle is much easier to aim