I use mostly ARs, but I just can't bring myself to complete the expert challenges on all the guns. I love the M16, AK47 and G3, but the M14 is just painful to use. I don't want to have to suffer through dozens of mediocre rounds trying to get headshots with the M14 when I could be owning people with the other ARs. Sure, I'd like to have a gold AK, but is it worth sacrificing hours of my valuable gaming time? Definitely not.
BTW, uberwoot in your first post you said "gold assault rifleS". You do realize that only the AK47 will be available in gold (as far as ARs go)?