I think it is pretty rare or those who have it aren't using it. I got one way early on for hitting rank 3 with Future War Cult. It was my first legendary primary weapon and I used it forever. But after TDB had been out for a month or so I broke it down, since I had plenty of better weapons. I regretted it, because I'm collecting all the FWC weapons. Luckily I just got another one on my most recent FWC package, so my collection is almost complete. I'm only missing a 331 Attack version of The Chance (hand cannon) and The Calming (fusion rifle).