What is really important? Accuracy and repeatability. If you get the "right" number, then it does not matter. Being able to check and verify that number over and over again gives you a bit of confidence. I have been using my setup for 20 plus years and checked it against known samples hundreds of times. When ever I find a thin area, I get real careful, but when going over a set of barrels, as long as the thickness is measuring above .025- .030 and is the same as areas nearby, I do not worry too much. If I find a area which is thinner than areas nearby, then I start looking for dent repairs and think about file work to blend the repair. If my setup is off .001-.005 I do not care as long as the walls are .025-.030+. I shoot several guns with barrels .020 as longs as the thin area is well towards the muzzle. But is is location and repeatability which my setup gives me consistently which is what I need. I know it is accurate because I have used it for decades and verified it with multiple other systems. We can not all be off the exact same way and same amount. Potential minor variations are not critical when you are talking about barrels not near a critical thinness. Mine are not. If I got to the point that .0006 was important then I would find a new gun.