The army has a new sniper rifle and I bet the Marines will field a weapon of similar caliber soon enough. I have grown up around the .300 win mag round my whole life as my dad and brother both hunt with them.
There is a reason the British are using a .338 round in their rifle. The .30 caliber magnum rounds have a superior ballisitic performance over the 7.62x51(,308) round. Put it this way at 500 meters a 7.62x51 168 grain match bullet which is close to what snipers shoot has a velocity of 1823 fps and has dropped 49.9" and delivers 1239 foot pounds of energy. The .300 win mag 168 grain at 500 meters is traveling at 2005 fps and has dropped 39" and has 1500 foot pounds of energy.
My dad tells a story about when he was in Vietnam about their firebase getting shelled every morning and they could see the spotter but no one could hit him with mortars, rifles, or machine guns even though they were shooting .50 cals at him. Anyway two guys show up at his position a sniper and spotter they hang out and get a feel for his habits and the sniper pulls out what looks like a commercial deer hunting rifle. They spot the guy the next day and bang the Vietnamese spotter ends up shot. Dad talks to the guys and they are using a Winchester Model 70 .300 win mag. Well when dad gets back from Vietnam he buys one. He has had this rifle for 40 years and it handles close in and range work he killed a big horn sheep with it at around 500 yards one shot.
I think the Army has a winner here and I hope the Corps adapts something along the same lines