Well, the score for the marksmen headshot was like 150, 160, 125 or thereabout since I was shooting from F to D (Conquest Large/64 player) on Silk Road with the last weapon EXP Sniper to try to get Variable and Bipod on it like the other snipers. So yeah, at the end of the round I was surprised it counted all of them together since like you I tried my damndest to get longer ranges and completely failed.
Just leaves the MBT and Assault Rifle kills for that, which will come with time.
I still gotta do the bombers and Jet kills (NOURAAAAAAAAAAAAAAAAAA~) to have all the China Rising assignments done. >_>