wncchester: "So I think another level would be required to level the gun and then use the bore sighter to level the scope. If the bore sighter has a leveler. Is this correct?"
Have never seen a level on a bore sighter or at least mine don't have them. The level in question is one from Midway that clamps onto a weaver base. I use two separate levels that I made up myself. On the bore sighter, I just use a scriped line to indicate a C/L. Then I look thru the scope and line up the crosshairs. I then back off and eyeball it to see where I'm at. I know that all boresighters have limitations and I can work with that. I also know that everyone has their own technique and I can work with that as well.
Be Safe !!!