Leupold does not make a QRW high enough to use on an AR platform unless you either use a riser(s) or your AR has a high rise upper.

Personally I don't like the riser blocks. It's just another link in the system that doesn't need to be there.

I use Warne AR specific rings. They are specifically built at the proper height for AR platforms.

http://www.warnescopemounts.com/maxima_rings.html