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Why??? that's what it's for.
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Because I want to have it sighted in correctly at 100 yards with whatever turns out to be my favorite ammunition.
I shouldn't ever have any ammunition that noticeably shoots off center in a rifled weapon. Vertical variation is tolerable since propellants and projectiles can and will vary.
For example, if I'm going hunting with my M1A and 175gr bullets, I know to click the sights down a couple notches. If I have a sidewind that I have calculated its effect on my bullet at a 200 yard shot, I use the windage adjustment.
If I am at a competition with 168gr match bullets, they will shoot differently than the 175gr hunting bullets. I move the vertical sights accordingly. But I don't ever want to have to worry if that windage adjustment on my rear sight is "true zero" or if it is some adjustment left over from last shooting session.
If I zero on the zero lines, I can see at a glance that my rifle is zeroed in and not adjusted for any situation that is no longer applicable. I want windage to be 'zero' when it needs to be zero.
I was told by somebody that each hash mark on the back of the receiver is 4MOA, so I will move the front sight to the left about 1/2 of one of those marks.