Note also that most cartridges are such that as the barrel's throat erodes away, bullets can be seated out further so their jump to the lands distance remains about the same. For example, with a .308 Win, after 4000 to 5000 rounds the throat's eroded away about 1/10th of an inch or so. There's usually enough bullet bearing surface length to let them be seated out 1/10th inch further than when the barrel was new.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master