I've got three or four so far just by spending time patrolling, they don't drop super often. When they do drop you will see a legendary icon of a gun pop up but it won't go into your weapons inventory, when you get back to the tower the gunsmiths icon will be highlighted and you turn it into him for rep and such. You don't get anything amazing for turning them in, it's kind of like turning in glimmer rewards to the cryptarch. Ive gotten three in the cosmodrome and one on Venus.