Re: changeing a .308 to .300 wsm
For one it takes a good smith. He will have to rechamber the barrel for the case and engrave the 308 win stamp out and engrave 300 WSM on it. He will have to open up the bolt face and install another type of extractor. On a repeater action, most will agree that the rails on the action the guide the shells from the magazine to the action/chamber will need to be reduced a bit for relaibly feedin the fatter cases. I think that about covers it.
Others will chime in on any thing missed or anyother trade secrets.
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.