Basically these two cartridges need to be buffed, since the .300 Magnum actually has inferior penetration compared to the .30-06, and the .338 Magnum has the exact same penetration as the .30-06.
But it is worth remembering that the .300 Magnum in real life is at least slightly stronger than a .30-06 (more energy), and the .338 is meant to be a BEAST.
Stats are as follows:
(Stats were taken from the store/inventory stats, I am not 100% they are the real stats, but they are the best I can find)
338 Magnum
Polymer Tip: 16 Expansion, 45 Penetration.
Soft Point: 63 Expansion, 22 Penetration.
.300 Magnum
Polymer Tip: 14 Expansion, 42 Penetration.
Soft Point: 60 Expansion, 21 Penetration.
.30-06
Polymer Tip: 13 Expansion, 45 Penetration.
Soft Point: 53 Expansion, 19 Penetration.
.308
Polymer Tip: 13 Expansion, 44 Penetration.
Soft Point: 52 Expansion, 19 Penetration.