For all intense purposes it appears that the 300 rum versus the 338 rum case dimension are the same yet velocity and muzzle energy with the same grain bullet are higher for the 338 rum...is there just more of the same powder loaded in the 338 rum?
NO.
The main reason for the higher velocity is the increased surface area on the rear of the .338 bullet. This is typical of cartridge design. Some other examples, the 6BR will make 150fps more velocity with a 55gr bullet than will a .22BR.
A 30-06 with 150's will make more velocity than a .270 with 140's and you need to drop clear down to 120gr bullets with the 25-06 to make up the speed.
al
