DRB can drop from random legendary engrams, as a reward from completing lost sectors and from completing spider bounties with a gear reward.
When it comes to having a good one, the two things you're looking for are Rifled Barrel/Full Choke and Accurized Rounds. If it has those two things, it's a keeper, the other two perks are entirely ancillary, but if you find one with Slideshot and Full Auto/Rampage in addition to the previously mentioned barrel perks and accurized rounds then you've basically got a godroll.
Range Masterwork is also preferred, obviously.
how do assault mag and full auto play in? Dont they increase the fire rate?
mine has full choke, assault mag, full auto, and range masterwork
i'm thinking my other previous one may well have had accurized, but it's not in my vault so evidently i sharded it :(
also isn't there a certain lost sector you can farm dust rock from for the other guy? maybe he should google that.