Weapons are like tools in a mechanics tool chest. Not one toll does all the jobs equally. The .300 Win Mag is going to have a much better first round hit percentage than a .308 past 600m.
The .308 is a great cartridge out to 600m but from 600-1100m the elements will start to challenge the best of "snipers" which make the first round hit sometimes very difficult.
A buddy of mine was stationed in Al Amarah and provided over watch for their compound. Their unit was mortared 2-3 times a day but the mortar teams would set up anywhere between 1000-2000 yards away. The unit was only armed with M24's (.308) and could only watch as these teams pulled up in a truck and set up position. After a few weeks of this a SF sniper team shows up with a Barret 82A1 and changed the whole playing field for the mortar teams.
The .308 is good for say 70% of all engagements. This leaves the .300WinMag MK13 for max range of 1400m, .338 Lapua Mag max range of 1700m(which has been in testing at the AMU for over 2 years now) and the .50 BMG with max range of 2200m.
The Army's/Navy's M24 Sniper rifle is a Long Action Remington 700 rifle in which the barrel and bolt face is the only element that will need to be changed out.
I didn't know the M24 was a long-action rifle. Clearly I'm speaking out of my pay-grade here
And I get what you're saying, I guess I was having a hard time thinking of a situation where an average sniper would need that kind of distance without the kind of planning that would require a special forces team, but that's a good example and clearly an example of why my opinion shouldn't mean much in the Army's decision making process..
Meh - like I said, I'm just playing devils advocate here - I know the .300 Win Mag is a good round, and I honestly don't have anything against it other than I have a complete lack of a need for one since I rarely even get access to a place where I can take a shot at >500yds