I bought a 6.5 Creedmoor when they first came out, the only ammo at the time was Hornady 129 SST Superformance. I've killed more than thirty deer with that round and they were all DRT except one that was chasing a doe and went as far as his momentum took him. Today I use the 143 ELDX which I think is a slightly heavier jacket than is optimum for thin skinned whitetails so I also use Hornady ELDM. I built a rifle with 7.5 twist to stabilize the 147 and I don't think I will go heavier because of lower velocity. Consider that the 6mm bullets in 243, 6ARC, 6mm Creedmoor and such are very effective whitetail rounds and heavier weight does not necessarily translate to better performance. I loaded some Sierra 123 TMK which I have not yet taken an animal with so I don't know how they perform on game. The Winchester 125 you mentioned will do just fine at 400 but a heavier bullet will retain more energy beyond that, practically speaking. Shot placement means more.