I’ve never used one on a gun barrel. Used to work in the gundrilling industry (gundrill means deep hole, not necessarily on gun barrels, that’s where they name comes from but we never made any gun barrels- except tank barrels) and so I’ve used them quite a bit. The stuff that we had was top of the line- so pretty pricey. As far as the cheap ones you guys are talking about- I can’t imagine the picture being particularly good. Personally- all I check is the crowns on my barrels.