[Optics] Vortex Strike Eagle 1-6x24 GEN2 AR-BDC3 with Cantilever Mount - $279.99 with code: WC2

You've got two things going at the same time when you're trying to look through a magnified optic (technically you've got at least four, but only two that I'm going to talk about)

Eye relief: this is how far your eye should be set back from the eyepiece to get that perfect setup where the image your eye picks up fills up the entire circle

Eyebox: this is how far up/down and left/right from the perfect dead center your eye can be and still have a useful image through the sight.

When you take those two things together (and add in parallax and FoV considerations) you end up with your sight picture. Sounds to me like you didn't have eyebox nailed down. That's usually the result of you not getting good and reproducible cheek weld on the stock of the rifle. For most rifles, you need to stick your face good and tight against the comb (top part) of the stock and move your head up, down, left, right until you find the ideal spot where a straight line from the center of the front lens through the center of the rear lens will also go through the pupil of your sighting eye.

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