Absolute pure fiction propagated by people who do not understand mechanical objects.To measure barrel wear, which I assume this is supposed to do, will only be determined this way IF you know two things.ie" the exact taper of the bullet ogive(most are not tapered but spherical,thereby complicating the formula)),and a reference point. This is akin to gauging a tapered thread or a tapered fit.Sticking a bullet into the muzzle does not tell you a thing.One of those old wive's tales we keep hearing.Most will say you are looking at the wrong end of the barrel if you are trying to see if a barrel is "wearing out". I'll bet it's near impossible to wear out the muzzle end of a barrel, unless it's with a cleaning rod.The other end of the rifleing can however wear out and affect accuracy.This is easy to verify by peering into the chamber and looking at the "throat".I once saw an uncle do exactly what your friend described about 30 years ago. I realized later that he knew even less about guns than I thought back then.