It gave you infinite ammunition. The only perk similar is String of Curses on Bad Juju. And that requires you to kill. Black Hammer only requires you to get 3 precision hits. Bad Juju is obtained after a quest line you have a random chance of getting. Black Hammer is obtained after killing Crota on Normal difficulty, a fight determined laughably easy now that many people can easily get 2 levels above everything in the encounter. The White Nail perk was Exotic tier. Bungie had 2 options: either make Black Hammer an exotic, or make it pull from your reserve ammo.