I was in the shooting world when Winchester brought out the the 308.
Some of the first comments from gun writers was that handloaders may have trouble duplicating the velocity of the factory rounds. Since they were all new rifles in that calibre, the first factory rounds may have been hotter loads.
The stories were that Winchester had to develope a powder for their factory loads and there just may not be a cannister powder out there that would be as good.
At that time the best cannister powder was considered to be 335. I forget what bullet weight it was ideal for, in the 308.
Except for lighter bullets, 335 would be too fast a burning powder to be ideal in a 243.