The same thing happens when you clean it down to bare steel between range sessions, but you learn a lot more during the shooting.
I've talked a number of barrel makers, some of them also metallurgists who make barrels with as good a reputation as any mentioned on this thread. Some of them firmly believe in things directly the opposite of several details from the break-in article. One, for instance, believes that cleaning too much of the carbon out of the bore is bad, that SOME carbon in the surface of the bore prevents copper or lead build-up.
Some would also find the admonition to not allow the barrel to heat up to any degree during break-in strange, since they do to great lengths to stress-relieve their barrels to prevent any "walking" with a hot barrel. I have seen some of those barrels at work when brand-new, and they do NOT shift POI even when shot screaming hot. And I also know of at least one brand-new barrel that took a national benchrest championship without being broken-in at all.
I have also talked so several barrelmakers who, as the article mentions, only come up with a "break-in procedure" because so many of their customers are convinced something magic happens when a barrel is "properly" broken-in, even though nobody can agree on the proper procedure.
