There are two main reasons for new barrels :-

1). Neglect causing rust and pitting and then the resultant lapping
2). DENTS! have these knocked out and the resultant polishing / lapping 2 or 3 times results in barrels out of proof. Some guns will not stand a re-proof having thin barrels already - result new barrels required...unfortunately the best handling guns have thin barrels to start with, its a trade off we have to suffer.

I doubt any of us would live long enough or even our forefathers and forefathers before that to physically shoot out a pair of barrels using just lead shot...

Jonty