Dont farm for it, trust me, dont do it. Everybody in my friend list had it, even the most casual players I've ever seen.. I wanted it and did the first dlc 'story' mission literally hundreds of times.. I was up to 11 hours of farming (in smaller chunks, dont worry) but got no drop.. then I gave up and a week later I got the bounty from eris, the first one dropped and the day after it dropped occasionally during patrol... I levelled it once and upgraded it. Currently it isnt worth it. You wouldnt use it for more than 5 minutes as it is almost outpaced by blue guns.. To upgrade it to exotic status it requires a special drop from crota himself on hard mode. If you're not planning to own crota on hard the very same day it is released by bungie, dont farm it.. you will probably get it eventually anyways..