I've noticed on the 360 d-Pad, the up button is reserved for something. What exactly is it?

Changing ammo types, it's that way on the PC as well even if you remap it elsewhere.

It is used to switch between ammo on your currently equipped weapon.

dif. ammo types. just hit up on the d-pad while a gun is out an it will switch to what other kind of ammo you have for that weapon.

Oh, okay. Thanks.