I have been looking at multiple posts about people just getting the bounty. And after viewing my grimiore and other peoples I see that I have over 10,000 kills with thorn but I haven't gotten it in y3. Most of the people who have the gun now in y3? Almost all of them have less than 1500.. no it doesn't reset when you get it, my friend who had it in y1 checked his grimiore before and after when he got the bounty when I mentioned this theory to him. I think that when someone from y1 had the gun, or used it a lot in y1, have an extremely low chance to get the bounty y3. I have only tested other people from the forums, so forgive me if you have lots of kill with it in y1 but still got it. But that still mighr be tied to just a really low chance.
I have less than 200 kills with thorn, by this logic I'm just extremely unlucky that I wasn't one of the first players who had it in y1 that hasn't gotten it yet