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I would first consider the role of this rifle.

If you are using your AR for home defense, I would just use the BUIS, or maybe a RDS. No need for a magnified optic unless you have a REALLY large home. In fact, a shotgun may be better suited for this role in the first place. If this is a hunting rifle, then an ACOG makes perfect sense, but you are unlikely to be "too close" to use the ACOG unless you are a much better hunter than I am. If this AR fills both roles, then a detachable mount makes sense.

That said, I have an RMR sitting on top of my ACOG. So I don't really practice what I preach. The Burris options mentioned above are also set up to mount a Fast Fire on top of them as well.