for u snipers that use a .308

well the .308 Winchester aka the 7.62*51mm NATO is the standard caliber in the military as a machine gun round and sniper round, was a common battle rifle round as well
since its already a military caliber, proven to be accurate and well suited for sniping, military ammo available
also there is an unlimited supply of .308 match ammo while i dont know of any .270 match ammo

also i dont know of any sniper rifles chambered in .270 Win either
all the guns ive seen chambered in .270 were hunting rifles not sniper rifles

i thout snipers in the military use 7mm

Sniper rifles were 30-06 as far back as WWII and maybe WWI then evolved into the .308 in VietNam. I've not heard of anyone using the 7mm for military purposes (maybe in Europe).

JeffVN

i dont think anone uses the 7mm
i think that the AI super magnum is available in 7mm Rem Mag but dont see why anyone would use it since it is also avaialbe in .300 WM and .338 Lapua
the .300 WM can do everything the 7mm RM can do then some while the .338 Lapua is in another league completely