Yep. Dennis is spot-on in every way.
Most (but not all) factory ammo uses powders handloaders can't get. This doesn't mean they're all that different, or "magic," just that they aren't quite the same as what we can buy.
The .300 H&H and .300 WSM are extremely similar in case capacity. In fact a gunsmith friend of mine ran an experiment with the same barrel rechambered from .300 H&H to .300 WSM and got essentially identical results with the same powder charges and bullets, and he was not only testing velocity but pressure with a strain-gauge system.
