I bought three new Colt ARs about six or eight years ago. A 6920 I equipped with a Mark AR Mod 1, 3-9x40mm with a duplex reticle. On an Expanse I put Mark AR Mod 1, 1-4x20mm, also with a duplex reticle. Good scopes with very repeatable adjustments. Whether or not these are still available, I don't know. If they are not, there is probably something comparable. Look at the Leupold website.

I know you're interested in scopes, but my third AR is the A4 version with a fixed stock and 20" barrel. I shoot it with the factory aperture sight. To 100 yards, this will work as well as a low powered scope if you're eyes are decent. Good luck-