As BO2 has recently been made backwards compatible, I decided to conclude the zombies ranking system mystery. As a bit of background information, I currently have 5 accounts with the shotgun rank, including my main account. A lot of people believed that it was about K/D, this is not true, although a high K/D can be a byproduct of the actual method to obtain those desired shotguns. The system is measured by your downs per round, or how many times you go down on a certain round. Generally, if you are hitting the high rounds, your kills will be high, which is why so many people ensured it was K/D. Proof that it's not K/D is TheRelaxingEnd, who managed to achieve shotguns with no kills.

So, with all this in mind, how the hell do I get them? For shotguns, you want to be hitting an average of 40-50 with 1 -2 downs. Now here's the catch, as some of you may have been playing on your account for years, and had no idea your downs affected rank, it means your dream of shotguns may be obsolete. This may mean you have to create a new account, please remember you can't get shotguns with 1 game on your account. For shotguns, I would say you need to hit 40-50 with 1 down, for about 35-50 games, this should be enough to complete your goal.
But I've been using my account for ages and want the rank on my main account? If this is the case you can try and consistently hit high rounds and see if your rank changes. If not then you may have to result into exploiting glitches to get rounds like 80-100+. Obviously I don't advise cheating, but I do feel there is some unfairness is the ranking system.

If you have any questions please comment down below.
Thank you