Right that the US-Army was using The 300-win ,270 ,30-378 wby and the 308 all back in '68 , so what new ?
The top US long range target shooters are about evenly split between 6mm and 6.5mm with only a few using 7.. like the 7mm Rem Mag or the 284. No one uses a 30 cal because the bullet Ballistic Coefficient is not as low so the bullet slows down more quickly from drag. in 6mm many top long range shooters use the 243 Win or the 6X47 Lapua. The problem with accuracy happens from wind drift and from a bullet crossing the sound barrier and the sonic crack throwing the bullet in unpredictable directions. A heavy for caliber and resulting low BC helps with both reducing wind drift and retained velocity at long range. a 6mm or a 6.5mm would have been the best choices, but the 300 Win Mag is not too bad.