Hi Bucky10,

Everyone has pretty much explained the differences in diameter of .277 (for the .270 Winchester)...and .264 (if you are referring to the .264 Winchester Magnum). And as a number of people have already pointed out the .270 Winchester is essentially a .30-06 case necked down to .277. The .264 Winchester Magnum as was also pointed out is a belted magnum. The cartridge case is the same as the .338 Winchester Magnum, except necked down to .264....(which is actually also 6.5mm). The case capacity of the .264 Win Mag is greater than the .270 Win. But in rifles of equal barrel length...the performance is nearly identical.

For much more info on either of these cartridges check:

http://www.chuckhawks.com/264mag.htm

http://www.chuckhawks.com/270win.htm

These two sites should give you a very good understanding of both cartridges. Hope this helps!