RE: .30-06 vs. .338 win mag vs. 300 win mag
The key to a successful kill is putting the bullet in the vitals, and choosing the right angle of the shot. A shot in the vitals with a .30-30 is always better than a non-vital shot with a .416 Rigby.....
For myself, I shoot much better with less recoil. A .300 win mag is maximum recoil for me. I can tolerate it, but three shots in a row with that gun takes it out of me. I can shoot a box of '06 ammo, and I'm fine.
A .30-06 is just as deadly at 300 yards or less as a .300 or .338..... that distance is also about it for me.
So in a nutshell, I'll keep the range of my shots within my shooting ability, and use a .30-06 that consistently shoots MOA. That makes me confident. If I can't get within 300 yards of an animal, then I have no business shooting at it. I'm sure that others are more capable at longer shots -- I am not.
Jack O'Connor shot plenty of N/A big game with a .270, and his wife shot plenty of African game with a 7x57 Mauser..... and today's bullets are much better constructed.....