The OP's wondering about barrel length for a hunting rifle, and as near as I can tell the typical 20" to 22" deal is as good as any.
The big problem with all this long-range hunting stuff is that absent a laser range-finder, the estimated distance is problematic. Next comes doping the wind; again: Problematic. Not many Mulligans in the field. Beyond 300 yards for most hunters, the odds of a poor shot go up and up and up.
I worked pretty hard at being quite good to 500 yards. But I've seen a helluva lot more bucks inside 200 yards than I did out beyond 300 yards.