Ive got 501 player kills and got nothing.

are any of them kills against people you were dueling or grouped with, because i'm reasonably sure those dont count.

They add to the counter for the medal. If they dont count towards it I wish they wouldnt do that. If thats the case I have no actual idea of how far I have to go to get 500 because theres no way of checking duels. I spent most of this week striving to get that damn medal and see what kinda cool gun I got for it. A bit disenchanting for something so hard to get.

Nothing other than the respect / fear of your fellow players. And isn't that all you can ever really gain in an online game?

perhaps deducting the amount of serco kills from your total would give you an idea of how many you have left since serco's are the only ones you really have to duel. it may not be completely accurate, but should give an idea.