The KNS sight posts tend to be longer than standard, which explains why you're bottomed out. No harm, no foul though as everything works.
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It's possible you may have bought a front sight designed for a2 and not a same plain front sight. Also keep in mind the height over bore with an ar15. Holdover is always needed within close range. Try zeroing at 50 yards. One of my favorite front sight posts is from blitzkreig. The post has a built in offset.