Re: why the .300....why not the 6.5x284
I think there are a couple of answers:
1) The larger diameter of the 30 caliber bullet leads to better terminal ballistic performance. The difference is notable, but on the counter side, the 6.5X55 Swede is used to take down moose today. Mortality can certainly be accomplished with a 6.5mm pill.
2) 30 caliber is a standard. Like it or not, for not only the US, but for her allies as well, 30 cal is a standard. We have a lot of 30 caliber barrels and 30 caliber bullets stockpiled. We can take existing barrels and simply rechamber them. For 6.5X284, the US would stand alone with 6.5mm. Others might convert, but 6.5 just isn't the standard.
3) Barrel life is not that great for the 6.5X284. As a matter of a fact, I would argue for terminal ballistics purposes and barrel life purposes, the original 284 WIN is superior to 6.5X284.
This is not to knock 6.5X284 at all. It will buck wind and punch paper or ring steel better than most. Barrel life is a huge concern because, as mentioned above. The folks who are getting >1,000 rounds out of them are setting back the barrel at 500 - 600 rounds.