I have only mounted one scope on a handgun (S&W 686, 8 3/8"barrel, Leupold scope) , but I did it the same way I mount my rifle scopes. First I place the gun in one of those plastic handgun shooting rests (I think it is made my MTM but am not sure). Then I placed a line level on the rib with a rubber band holding light tension on the level to keep it in place. I eyeball the level to see that it appears level and that the rubber band isn't skewing it to one side. I have a blaze orange string thumb tacked to the ceiling with a sinker on the end that I use as a plumb bob. I then look through the scope and line up the vertical crosshair with the orange string. I keep checking to make sure the level bubble is still in the center as I make adjustments and tighten. There is some parallax in the scope at this range, so I can move my head left to right so that I can place the crosshairs right beside the string and see that they are parallel. I also own the Segway device, and I sometimes add that as another means of checking my results, but I find that it isn't as easy to use as the plumb bob.