I just purchased a Rock River Arms Flattop Riser for use with a rifle scope.  The scope is fitted with Leupold Quick-Release Weaver Rings.  I installed the Riser on the my AR flattop and then tried to install the scope on the Riser.  When I tighten the scope rings I noticed that the scope was still able to slide back and forth on the Riser.  I measured the Riser and found that it was 0.800 inches wide.  The standard for Picatinny Rails is 0.835 inches.  The Riser rail is almost 1.0 mm too narrow.  The clamp on the scope rings bottoms out on itself before it gets a good hold on the rail.  As anybody else had this problem with the RRA Riser?

The base/female portion of the flat-top riser base is M1913, but the top rail is a basic weaver pattern, and not M1913.  As "Weaver" is a generic term applied to just about every base that has the diamond-looking cross-section, different rings fit different bases differently.  I have two of the riser bases, and they measure identically.  If I were to mount one to the other, the top one would be terribly loose and flopping side to side, as well as front to back.  I used a a pair of Millett rings one one, and the integral ring from a red dot (one of the ..gulp, Tasco PDPs...don't remember which one) on the other, but did have trouble with others being loose or tight, depending on brand.
Sluggo

Talked with a gentleman at Rock River Arms today.  He confirmed that the upper rail on their riser is not a Mil-Spec Picatinny rail.  He indicated that other people have had trouble with various scope rings on this riser.  He mentioned that they are going to replace it soon with a riser of their own manufacture (this one is purchased from another company).  He said their new riser would be Mil-Spec top and bottom.  Does anybody know if the riser sold by Fulton Armory has a Mil-Spec top rail?