I usually set a rifle up in a rest on the kitchen counter and then level the rifle. Then I hang a plumbbob from a tree in the yard and line up the vertical crosshair with the plumbbob line outside. Then on a rifle I'm using for target I install a level similar to the one you're looking at while everythings plumb and then use it for leveling my rifle at the range rather than the bubble on my rest. It's much easier and more accurate that way. The one in the picture looks a little easier to damage than the ones that I use. They're available from Midway or most any large shooting supply house for $20-30.