For making consistant 400 yard kills on deer,three components are required:

1)a cartridge with sufficient energy to reliably kill a deer at 400 yards

2)a rifle and load combination that is accurate to 400 yards

3)a hunter that can consistantly place a bullet in the vitals at 400 yards

The first two are fairly easy to find but the third component is usually the difficult one. Most people simply will not spend the time and ammunition necessary to develop and maintain the skills necessary to be consistantly accurate at 400 yards.