The difference between the .264 and the .270 are the bullet diameters. There are several case designs for different bullet diameters. For example the 7mm(.284) diameter bullets have gained many factory cartridges. To name a few...7x30 waters, 7mm-08, 7x57, 280 Remington, 7mm Rem. Mag, 7mm Weatherby Mag, 7mm STW, 7mm, Rem. Ultra Mag., as well as a few others that I left out, and some wildcats also. All of these have the same bullet diameter but much different case designs. The bullet diameter has no direct relation to the size of the case as a standard. The .270 was a design off of the .30-06 Sprg case. The .264 is a case design that Winchester developed to shoot smaller bullets very fast. Hope this helps.