Ive dismantled a few, i still have one that's completely untouched with field scout. I don't think it's possible or even relevant to label a gun the rarest in the game, but if i had to guess "the conduit" wouldn't be it. Gun rarities differ from one player to the next, ive seen so many talk about thunderlord or hardlight (obviously before xur sold it ) as being the rarest guns but I've gotten multiple of each, i on the other hand find ghorn/hawkmoon to be my rarest but others have had multiple of each. The game chooses what's rare for each of us