So I have a question. Have you done any long range shooting before and know that you can in fact shoot the distances you are describing? There is nothing wrong with limiting your ranges to someting more manageable with a cartridge that fits the platform. The big heavy bullets out of the .308 like 185's and higher is a very capable cartridge out to 1000 easily and even further once you get a little practice under your belt. I would suggest using your current action as more of a learning platform and build a new rig for the extreme long range work you are wanting to do latter on. Save the money you would spend on bullets, powder, dies and a new barrel and buy bullets and powder for the .308. Practice, practice and practice then move up into a better platform and cartridge as time and money allows.