I think the .270 is .277 inch and 7 mm calulates into .276 inch (but I've seen it written that it's a .284 inch, and I don't understand how 7mm could give .284). Either way, virtually the same - they both round to .28 to the thousanths of an inch!
The 7mm bores do indeed use .284" bullets.Rounding to the thousandth, the 270win uses a .277" bullet and the 7mm-08 uses a .284" bullet for a difference of .007".