Hmm...
I must say that my point of view comes from a different army than the US, and maybe that's the reason I am saying 300 rounds and over is too much.
If you give a soldier loads of ammo and other stuff, he/she will not be able to move fast, right?
Obviously the army where I served concentrates more on fast and flexible moving tactics (we have to do so because we have a small army and relatively large country).
If I had those, let's say 12 magazines plus the other stuff like anti-tank mines (10-20 kg each, could be 2 with me), hand grenades and the backpack, I could barely walk with that load. I will need the mines, so what do I drop? The magazines, of course.
And I think the US soldiers agree on this. There's ammo trucks with you because they are ment to do the hard job of getting those rounds to the battle field.
That's the point I'm trying to show to you. It's better to let the men run fast than slow them down with unnecessary amount of ammunition.