Like Mark Elliott, I store mine horizontally on my secure den walls using two vertical stringers per row of guns. The stringers are fastened to the heavy molding strip at the top of the wall and are screwed deeply into the top plate of the frame construction. Each stringer is 2" x 3/4" and as long (tall) as needed, are oak, and have diagonal rows of pilot holes drilled at 1/2" horizontal spacing and 1/4" vertical spacing and at an angle so the screws supporting the rifles are angled upward. (JTR will post photos for me soon so you can see what I mean.) The screws are placed appropriately to level each rifle, and are sheathed with soft plastic tubes to avoid scratching the guns. Because I live in an earthquake zone, I fasten each gun with bungee cords (as are sold for fastening tarps), and also screw the bottom of the stringers to the wall. Since each stringer is set away from the wall by 3/4", there is room behind the guns for other flat display items if desired (like my Navajo rugs or pelts, etc.). The screws can be repositioned when I want to change guns, and the walls don't suffer at all from multiple extraneous screw holes.
The multiple stringer holes can be drilled using a drill press and a guide clamped to the angled table to keep each vertical row in perfect alignment. Laying out the vertical and angled lines with pencil and then center punching helps. Since I have quite a few racks, however, I made a drill-block jig out of 3/4" steel and sequentially drill about 30 holes at a time in perfect alignment using a hand held battery drill. This display rack system is safe for the guns and the walls, and allows me to enjoy the collection fully. The bungees are tight, but slip off the screw heads without difficulty when I want to fondle the guns. This system has been very satisfactory for my collection.