Ideally, the barrel should be absolutely true, the flats on the receiver and on the front sight base being absolutely parallel. It the front sight is not at 90 degrees to the receiver flat behind the rear sight, the eye will notice it. Also, with the rear sight centred, the front sight will noticably off to one side, when the rifle is zeroed for windage. I suspect that most shooters would notice + or - a degree or two deviation. At the OSA M-14 clinic we indexed one barrel that was badly off index, to the point that the front sight was way over on its base. The barrel was several degrees off, and it was very obvious when aiming the rifle.
Some folks use steel parallels on the receiver flat and front sight base when the barrel is being indexed. I prefer to use a machinst's level, set it on one flat, when the level is on the other flat with the bubble in the same position, I assume that the barrel is indexed.
I do not know why rifles are allowed to leave the factory with the barrels improperly indexed. With the systems available at the factory level, the barrels should be dead true.
Incidentally, Winchester 94 barrels are properly timed so that they will index correctly, as are No.4 barrels.
Some folks use steel parallels on the receiver flat and front sight base when the barrel is being indexed. I prefer to use a machinst's level, set it on one flat, when the level is on the other flat with the bubble in the same position, I assume that the barrel is indexed.
I do not know why rifles are allowed to leave the factory with the barrels improperly indexed. With the systems available at the factory level, the barrels should be dead true.
Incidentally, Winchester 94 barrels are properly timed so that they will index correctly, as are No.4 barrels.