The biggest question I find on these forums is "Do my kills count if the players drop below 6?"
The answer is, yes, they do count. Your "Seriously..." and weapon achievements will still count, no matter how many people. Even if the host quits. As long as you stay in the match until it is over, you'll get you're kills.
People are confused over this because if you go look on the leaderboards, it does not count these kills or points, because they do not consider these as legitimate. But you, as a gamer, still receive your kills.
Got it? However, this game's achievements are still really messed up, so I don't know if they'll unlock at the right time for you. I mean, come on Epic, how the hell do you screw up your achievement system, THIS badly? lol