RJY66, as I understand it (I'm sure Mule Deer can provide a more clear explanation) the 4:1 rule said if a cartridge uses 20% more powder than another, smaller cartridge of the same caliber/bullet weight, it will only yield 5% more velocity, as 20/4 = 5. So when looking at the .308 Win (approximately 40 grains powder used) and a .300 magnum that can use 80 grains of powder, though the magnum holds 100% more powder, it will yield approximately 25% more velocity (ca. 2800 fps vs ca. 3500 fps with the magnum).
