I built a couple 6.5 grndel ar 15s last year, (some may remember helping troubleshoot both builds when new) both the cartridge and custom barrels were completely new to me. I continue to be impressed with how little of copper comes out when I clean them. Almost none. I am using patch out with accelerator and get very copper. Is that due to better or smoother barrels? Or is that because of the bullet going 500 fps slower than I am used to? Maybe both? One is an xcaliber, one a faxon, if it matters. Fwiw, from what I have heard the xcaliber was a pretty good quality custom barrel and the faxon was more of a budget option, but still usually better than a factory barrel. From accuracy and how easily they clean the faxon is right there, both seem to shoot great.