A lot of people have been spreading information saying that all you have to get for double shotguns is a kill/down ratio of 200. This is false. I have a k/d ratio of above 350 and have had shotguns for a few days. Earlier today I was messing around in Die Rise and lost 5 or 6 games in a row before even making it to wave 10. After the 5th or 6th game in a row I lost at such a low level, I noticed I got deranked to the machete. My k/d was still 330+ at this time.
Since my k/d was above 330 and I had the machete, the rank is clearly not just about having a 200+ k/d. It also seems to take into account how consistent you are with making high waves. I played a game immediately after deranking and got to wave 60+, earning me back my shotguns.
TL;DR --- Kill/down ratio is NOT all that matters in your zombies rank icon!