You are correct there is very little difference. In fact any of the cartridges where the same bullets are loaded in the basic .308 and 7x57 cases show the same thing. In order to get more velocity out of the 6mm you have to load it to a higher pressure, and then you won't get an increase of 25 yards in trajectory. Check 'em out in the manuals. .260 Rem, 6.5x55/57; .7mm-08, 7x57' .243, 6mm. there ain't anywhere a 200 fps muzzle velocity difference in max loads with the same bullet.