Free: Contests & Raffles.
As far as rifling erosion vs throat erosion when in the Marines as an armorer I replaced far more barrels due to rifling erosion than throat erosion. Of course military rifles will wear out faster than hunting rifles by mere number of rounds fired. Most are estimated to have a barrel life of 10,000 ends at production but that's arbitrary you could see far more or less than that.
The reason there are so many Ruger upgrades is because they're necessary.