I use a couple 12" blind flanges which measures 19 or 20in depending on the #rating of the flange (150# or 300#). They are mild steel, however, at 2" thick, even the 300 & 338 RUMS don't do much in the way of damage. Here are two pics thru the Optics, top pic is @ 550yds thru a Leupy VH reticle @ 25x, Bottom is same target @ 1060 thru NF NP-R2 22x.