When you ask flattest shooting without specifying the use, that makes it too abstract a question. The others are right though who mention there are different flat-making factors and that you can't have all of them. Two more that got left out are the law of diminishing returns and barrel wear as you get to the upper end of what you can cram into a case behind the bullet. Another is accuracy which will have different parameters for shooting moose, gophers or paper targets in different types of competition. Although a bit smaller than your 270 minimum, it's no accident that 6.5/284 rules the long distance roost. In addition to factors of efficiency and accuracy there's also a great choice of high sectional density long distance 6.5mm bullets.