Note that a barre's bore axis does not point directly at some place above the desired impact point equal to the bullet's drop from bore axis at that range when the round fires and the bullet does strike the desired impact point.
The bore axis moves a bit in different directions while the bullet's going down the barrel. How much it moves depends on how the rifle's held, where the recoil axis is relative to the butt plate's position on the shooter and how much recoil there is. Two people shooting the same rifle and ammo typically have different sight settings for a zero at a given range; it happens and is most noticed with scope sights.
So, a real accurate bore sight by any means just gets bullet impact close to point of aim, not perfect, so you gotta shoot the darned rifle and ammo to get it right.
Ever wonder why the front sight on handguns is higher above the bore axis than the rear sight? And double barrel elephant whompers' barrel axes cross 10 to 20 yards down range to zero at 40 or 50 yards.