In the real world I would keep it simple:
Primary LR Rifle: AIAW L115A3 (suppressed of course). It is currently fielded be several of our allies.
Chambering: .338 Lapua
The .300 WinMag is a great cartridge but not when compared to what a lot of other countries are using, we need to be able to at least match them on effective range.
I have had a .338 (Edge and/or RUM), and fired many more, for several years now and know what they can do.
Scope: S&B PMII 5-25x56
A lot of great newer scopes on the market but this is still the standard that others are compared, and for good reason.
Spotter/Secondary Rifle:
A lighter and more accurate version of our M110.
Chambering: leave it 7.62x51
More than capable of medium ranges and quick multiple engagements, and the biggest plus...ammo all over the place (on probably any battlefield!). You would understand why I say this if you have ever ran out of ammo in a gunfight.
Optic: I like the size and power (magnification) range of the 3.5-10 MK4 we have on them now. However this scope is 90's technology and needs to be upgraded. Bare minimum of FFP and matching turrets/reticle...there are better choices.
Anti-Material Rifle: get rid of that **** Barrett and get something made for precision, the McMillian Tac-50 that some of our allies use would be great. On top of that, quit giving us delinked M2 ammo. We need some match grade!
Optic: Another Schmidt on this one.
Notice I mentioned a couple of rifles already fielded by other countries. That's because they take it more serious than the U.S. and we are just as of late playing catch-up. Plus these have been fielded for years and are proven performers.
A custom rifle would be great, but has no place where being the one and only can turn into being the one and lonely if repairs are needed...and they will be. When someone says anything to me about thinking we need X or Y cartridge/caliber I may agree about it's perfoance but disagree on it for ammo reasons. In the darkest places on earth, I don't want to be the only fool with a rifle that nobody else in the country has ammo for! If you don't think getting ammo is already a problem then you are fooling yourself.
In a fantasy world I would take my .375 Snipetac (with my suppressor) and maybe an upgrade from the NXS I have mounted.