I'm late to the party, but I always start these things with....what is shot out? Shot out, to me, is when for a single load, the rifle shoots worse than I will accept.
Finding a load that shoots best is extending that life.
If I will accept 1.5" accuracy from my hunting rifle and it shoots 1" with my best factory ammo, it is 2/3s shot out. If I make ammo that shoots 0.75" groups, now it is 1/2 shot out.....Over 100's of shots, it will degrade for the same load. When you get to 1.25", you might play with the load and bring it back to closer to 1"....you just gained useful life. At some point it will not shoot under 1.5"...no matter what you do. That is shot out.
You can change your acceptance level from 1.5 to 0.5 or 3 and those will affect your amount of shot out.
The final thing to remember is that barrels are consumables. You can buy another for about 25% of rifle cost.