Ah yes! The military ammo debate! It's only been going on for over 100 years.
FWIW, if not for the intransigence of the US Army Ordnance Corps (note to BHO, that is pronounced "Core" not "Corpse"), we would have had an intermediate power combat rifle round somewhere between 6.5mm and 7mm back in 1955. The .308 Win would have died a natural death (no real difference between the .300 Savage and .30-06 to warrant an existence), as would have the .222 Remington Magnum (which became the 5.56 NATO with some minor mods).
The British did extensive ballistics tests at the end of WW2, since they clearly had the most obsolete cartridge of the day -- .303 which was introduced about 1885 or so. They tested the (then) new .30 cal Russian (7.62x39) and the 8mm German (7.92 Kurz) and determined that an intermediate power cartridge of about .280 caliber (7mm) gave the optimum performance for a combat round http://en.wikipedia.org/wiki/.280_British
. FN Engineers, who had spent their vacation from German occupation in the UK, were involved in much of this work, so they went back to Belgium to design a proper rifle for this round -- the FAL.
The US Army wanted the .308, but the British felt it was too high power to be controllable on FA fire. The US Army insisted on .30-06 performance out to 500m or so.
When the US Army nixed the .280 British, Saive and his team had to significantly redesign the FAL to take the much higher power .308 Win round. After stuffing the .308 Win down NATO's throat, the US Army used it for about 6 years in a combat rifle (M14), and then we went to the 5.56 NATO because the .308 Win was too high power to be controllable on FA fire.
Some other notable 6.5mm cartridges:
6.5x54MS (Dutch & Greek military) 1903
6.5x55 (Swedish military) 1891
6.5x50 (Japanese military) 1897
.257 Roberts (sporting)
.264 Win Magnum (sporting)
.260 Remington (sporting)
6.5 Remington Magnum (sporting)
Some notable 6.8mm cartriges:
.270 Win (sporting)
.270 Wby Mag (sporting)
.270 WSM (sporting)
6.8 Rem SPC (? wannabee military)
Some notable 7mm cartriges:
7x57 (Spanish, Chilean military)
7mm Rem Mag (sporting)
7mm Wby Mag (sporting)
7mm WSM (sporting)
.280 Rem (sporting)
.280 British (dead - wannabee military)
Note to Mods - this probably belongs under "Ammo & Reloading"