I have a Flattop Riser that I purchased from Fulton Armory.  I have a scope with Leupold QR Weaver rings that a tried mounting to my AR using the riser.  The scope has worked well on my bolt rifle for years.  When I mounted the scope on the AR using the riser the scope is pointing about 30" low at 100 yards.  I am fairly certain the the scope rings and the AR flat top are OK, so it must be the riser that is out of line.  I figure I need to shim the riser about 0.030" to straighten things out.  Since the riser engages the flat top for a distance of about 5 inches what would be the best way to shim it?

Can't turn the riser around backwards?

Not sure which model you have so I don't know if it's a reversible symetrical design.

There are tapered risers out there for AR15s though with MOA from 15-20MOA of taper.