No, it does not have to be a certain game mode at all. I recently just got the pistoleer challenge completed as you mentioned while playing in tdm. The game is extremly buggy and Ive been cheated myself out the hail mary challenge afew times over. Did you get 10 headshots with just pistols or did you get some headshots with other weapons? Like did you get 7 headshots with a pistol and 3 with a smg? If not just accept the fact that you will be screwed out of challenges you did complete but it didn't register. Hope this helps.