Good gentlesirs,

For my edification, if I wished to build a super long range rifle, for shots, at targets, to 3000M and not using the.50 BMG what would you suggest...Some have been using .338/378 etc. to 2000M...Would using a larger case get me the range and power necessary? I have seen a .338/416 Sniper in the wildcat tables...Would using a .404 or .505 case work out? It would probably have to be necked down to .33 or .35 to carry that distance or could I go to a .30 or .31? Any suggestions, advice would be greatly appreciated.