Parallax is an error caused by your scope's focus not being adjusted to match the target distance.
You must adjust the parallax/focus before you can expect precise bullet placement.
Lower cost scopes may not have parallax adjustments.
These are usually set for 100yds.
Some scopes will have actual range markings on the focus knob, but most will make you figure it out by eye.
To adjust the parallax, start by focusing the image as you look through the scope.
Then get the rifle on a solid rest with the reticle on-target.
Now move your eye up and down or left and right WITHOUT disturbing the rifle's aim.
If you see the reticle moving around on-target, that is a parallax error you are seing.
Continue to make small adjustments to the focus until the reticle does not move on target.
Now you are ready to make a precise shot.
Some scopes may go slightly out of focus when the parallax is set properly.
This is not desirable, but it's more important to have the reticle parallax-free than perfect focus.
Go with perfect parallax over focus every time.
On my nightforces, I usually find that if I find perfect focus, I need to roll the top of the knob about 1/16" forward to find perfect parallax.
The more consistent your cheekweld on the stock is, the less parallax error you will see.
This is one reason that a proper consistent cheekweld is so important.
A scope with no parallax adjustment will require you to be much more diligent in your position.
The more finely tuned you get the parallax, the more sloppy your position can be.
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Last edited by ar15barrels; 04-06-2010 at 11:08 PM..