I thought you could only get this from death matches and the such? From killing other players. I know for a fact that I haven't head shotted 500 other players... I tend to avoid other players during Free Roam, and the ones I do harass, I do so with an advantage, like a helicopter. I was just sniping some random beach goers after training my stealth skills on the dock. (Currently 40/100, pretty hard to raise that skill.)
Edit: Yup, just checked my stats, only 215 player headshots. Still got the 500 head shot tattoo though. I have 5,804 NPC kills, and 1,993 NPC head shot kills though... Hmmm, guess it's bugged? Also, lucky you guys if you ever find me, apparently only 20,774 shots out my 68,567 have hit someone, so, very likely to miss it looks like.