How many kills needed to rank up in weapon mastery?

Now I had to kill 81 to rank up from 44 to 45. Is it because I am playing in Casual Breakthru with bots?

Yes, bots are providing less XP than real players. But, it still can be beneficial if you kill many of them, so you will get more XP in total than in the regular game mode (with less kills).

Thanks a lot bro ;)