I've tried reaching 50k legit alone and have hovered around 30k. It took me a month. It is very near impossible playing alone unless you leave every game after killing a few people and farm bounty(This way looks bad IMO). You must have played with a very generous teammate OR have friends that will bounty you because they don't care about bounty. And the most reasonable way to get 100k bounty, an alt account.
~I tried so hard and got so far... But in the end it doesn't even matter~
(This post isn't meant to be rude in any way. Just giving you my experience and P.O.V on keeping bounty. I myself was once a bounty hunter.)