Hey mate, the fastest way is to do all bounties on all 3 characters. Get a rocket telemetry (gunsmith, but it's not necessary) activate it right before handing in the 10 bounties and make sure the weapon is equipped and nothing else that can be levelled up including armour or you will lose some xp to that item as well, then do the weekly or daily mission within the time limit. Do this for all 3 characters and it will take around 30 bounties to max. If you don't have 3 characters you will have to wait until daily reset and go again. Hope this helps.