Barrel break in

[JSnover]

When folks do not do a proper break in, and cleaning, I get their guns in for a re-barrel,

Would that be a re-barrel because they think it's shot out, or because they ruined it by not cleaning? I have more than thirty barrels in my safes dating from 1890 to 2018 and I'd be surprised if any of them were 'properly' broken in but none have been ruined.
Barrel break-in is somewhere between myth and science, or maybe a combination of both. Some people get it done in an afternoon and others take years, giving the barrel six-to-twelve months to cool off between groups.

[John Taylor]

Have seen more than one barrel set back and re-chambered because the throat was gone. Flame cutting on the throat is a problem for guns that have been shot a lot. One of the reason for going to stainless steel, you can't cut it with a torch so the throat does not go away as fast. The 4140 steel used in many rifle barrels is much tougher than the 416 SS but depending on the cartridge a 4140 steel barrel may have the throat problem in as little as 2,000 rounds.