I tried a PA Slx 1X microprism at first on my 556 RAR mounted in a Sharp Bros Heatseeker chassis. Worked really well for my purposes - a 200-300 yard rifle. Good eye relief and a distortion free sight picture plus I really like their ACSS reticles.
I then decided I wanted a little magnification and replaced the 1X with a 3X in case I wanted to stretch the shots out a little further. I like the increased magnification but I noticed a couple of things. The first is the shorter eye relief - 2.7 inches. PA doesn't list an eye relief for their 1X so it must not be a factor for that scope. In my picture below, you can see that if you use the factory stock, you're going to have to find a mount that brings the PA closer to your eye or really lean into the scope. It's not a problem with the Heatseeker since the LOP is about 1.75 inches shorter.
The other thing that's noticeable is the small amount of distortion around the edge of the sight picture compared to my PA 1X microprisms which have no distortion as far as I can tell. But the center of 3X sight picture is clear and distortion free. Personally, I can live with that little bit of distortion in exchange for the increased magnification.
Now if you really want to stretch it out further, you might try something like this setup - a 556 RAR with an Arken 4-16 scope. DuneShoot posted these pictures over in the Bolt Action Rifle section of the forum. He has some real promising results with a stock RAR with no mods other than a scope and bipod using factory IMI 77gr 556 at 100 yards.