Ever wondered why a weapon is 'ranked' higher than others? This should help!
When you sort weapons by Rank, they are first sorted by star level (obviously).
From there, they are sorted by the date the skin (as a whole) was first introduced into the game, from most recent to older.
Therefore, the 'rank' of a gun in your inventory doesn't indicate it's relative performance.
For example, let's say we have a bunch of weapons all at the same star level.
So, taking this into context... let's take a look at the UNK CQB Nightcrawler... it's a relatively new gun... however, the Nightcrawler skin was the first skin introduced into the game (other than Veteran, obviously)... and so this gun will be 'ranked' lower than the same-star UNK CQB AWD2016, or the same-star AG 10 Tiger, etc.