Leupold makes scopes specifically for the AR rifles. They're called (not surprisingly) the "Mark AR" series. The 3-9x models have a trajectory-compensation feature. Zero the rifle at a given range, adjust the scope to read that range on the elevation dial, and we can then set the scope for any practical distance and hold dead-on at that distance. These are calibrated for the 55-grain bullet at 3100 fps, and in my Armalite AR-15, the system works perfectly. It's my understanding that other loads can be utilized by substituting different turret dials.
I also have the Leupold Mark AR 1.5-4X scope on my Mini-14. This one does NOT have the trajectory-compensation feature, but it's a very fine glass indeed. Just last week-end, I was using it to pick-off ground squirrels beyond 100 yards with little difficulty.
Both of these "AR"-series scopes have now reached the over-500-rounds mark, and both are still working fine.
The 3-9X cost me just over $300 on special order at a local store; the other came from Cabela's as a gift, and I don't know what it cost.
I recommend them both highly. I wonder if Leupold has engineered them with the slam-bang actions of the autoloaders in mind? I would hope so.
My AR-15 is a "flat-top" version, and the scope is mounted quite low, but I mounted a detachable DPMS front sight just so the rifle would look better. That front sight is INVISIBLE in the 3-9X scope; it simply does not appear in the field of view at all. I now note that the intent is to shoot to 500 or 600 yards, so at least the 3-9X will be indicated. In such a case, don't worry about the front sight. Mount the scope at a comfortable height and carry on.
