Only 1/3 of the time I do the Round 15 Easy on Town thing. I mostly play Buried and get to 30+
BUt even round 30-40 you only get about 1000-2500 kills and it takes 2 hours most of the time to get to 30 and another hour to get to fourty so there is no way thats how you got all your kills
Everyone has their own opinion, here is mine based on observation...
K/D has almost nothing to do with it, nor do revives or high round scores, or any of the stats they let you track directly...
The primary factor that determines rank is your AVERAGE HIGH ROUND ACHIEVED. If you get to round 50 on a map, but you died on round 10 the try before that, your average high round would be 30 for those 2 tries. I got my shotguns when my k/d was about 165, and maintained them while my k/d dropped below 150 (due to playing more grief, which does not factor in to ranking). I finally lost them a few days after Origins came out, and over those few days my average high round was horrid I'm sure, though my k/d never really slipped much. I have since raised my k/d back up a bit, but still no shotguns.
Additionally, I have a friend that plays a lot of grief, but when he plays survival he does not mess around. His k/d is only around 70/1, but he has had shotguns since nuketown dropped. I literally have him beat in every single stat line, but he has shotguns and I do not... why? Because he does not play survival with randoms, he does not do perma-perk "prep" rounds, he does not quit on games. His average high round for survival is probably extremely high, hence the rank.
There is likely a minimum kill threshold before it will let you rank up, but I think you are well past that point. Just play zombies efficiently, don't quit, bring a team that won't quit, etc... Hope this helps!
your friend was 70/1 on career stats but what was his co-op stats. prob in between 160-200 kd? your ranking is based on co-op stats not career
Currently testing this theory. I made it up to shotguns and then lost them doing easter eggs. I lost them when my KD was 180 and made it drop to 177. Then, I played a solo game and got it back up to 180 where I was rewarded with my shotguns again. Once I got a single down, I lost them and then when I got 100 kills to match that down, I got them back.