Is the Vault of Glass Templar drop Praetorian Foil fusion rifle the rarest gun in the game? I had one and dismantled it because I didn't know what ascension was at the time.
2 Answers
No, there really isn't a rarest gun in the game. That is to say, there are no known (official) drop percentages for any of the weapons.
Praetorian foil is a pretty rare drop though, as are most of the hard mode VoG weapons(Praetorian Foil, Praedyth's Timepiece, Fatebringer, Corrective Measure). This is purely anecdotal, as my own experience.
2The Necrochasm is the rarest weapon in the game because of the very low drop rate of the Crux of Crota in order to upgrade from Eidolon Ally.
There have been no notices about the drop rate of the Praetorian Foil, and as such it must share the same rate as other legendaries in the Vault of Glass. Don't worry about dismantling it, you can easily obtain another by running a few weeks of Vault of Glass.
