What does a barrel look like when looking through a microscope? depending on how the barrel was finished will depend on how the bore surface looks. some barrels are not finished very well and have little metal jags sticking up from the manufacturing process, these little jags peal copper off the jacket as the jacket passes by, that copper stays in the barrel and can build up. if you clean that copper off, the next bullet passing by will lay those jags down a bit more and then a bit more, if you don't clean the copper off then it builds up and builds up and can affect size and shape of the next bullet passing by. Great barrels are often lapped or have extremely fine finishes and have no little metal jags stick up so the first shot lays down little to no copper. As you can see it totally depends on the barrel. In fact no two barrels are a like. what affects accuracy is copper build up in the barrel, you need to do all you can to mitigate that.