If you have a continuous rail, low rings will not fit*. I used the mount and it's solid. The large rings appear to work on paper. Don't listen to the internet ninjas.
* Unless you have a dremel and willing to use it. You might have other issues if you're thinking about doing it.
I had the same issue with medium rings. It touched, though barely, too close for repeated usage.
Well I ordered the medium, hope it works out since I too have a flat rail AR