Re: 338 winmag or Lapua?
4000 rounds is very optimistic for the 338 WM. I think closer to about 1/2 that is more realistic.
If you can afford the LM, that is a far better 1200 yard cartridge. If you cant, then dont sweat it.
You are right, the 338 LM brass is phenominal brass and the LR potential of the cartridge is great indeed. Plan on 1200-1500 rounds +/- of good barrel life with the LM in a good SS barrel.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.