I bought one of these a while ago and just got around to opening it up and trying it out while mounting a scope on a Ruger 10/22. I hadn't yet messed with it because it seemed like it would be pretty inaccurate since you'd have to measure how far the middle of the scope is above the middle of the barrel to get it aligned. Anyway, after opening it up and trying it out, I was surprised to find that the "target" in the boresight doesn't move based on the height of the device - the height only has to do with the backlight provided to light the boresight target. Wow. Not really sure how the boresight target remains in place as the height changes; magnetic, perhaps? Anyway, I was pleasantly surprised. I know boresighters are generally a waste of money, but I was surprised at how this one works. Or maybe they are all like this; I don't know - it's the first one I've used.
For the record, it's this one: http://www.leupold.com/tactical/products...ed-boresighter/
For the record, it's this one: http://www.leupold.com/tactical/products...ed-boresighter/