Bungie has unlocked ‘The First Firewall’ quest and the requirements for doing it is that you need to turn in 4 fusion rifles to the gunsmith. These fusion rifles are something you find while killing enemies and you must have gotten a bunch of them. They usually have names like FR003RAS etc.
The mission is what gets you started on the Sleeper Simulant questline but you need to do a ton of stuff to actually get it.
Eventually you will get a core that lets you input codes. Here are the codes.
1 is the top row, 2 is the bottom row. Input the following code four times and you will be given the next quest which is to reactivate a fusion core.
Do the warsat public events on Earth, Moon and Mars and dismantle a legendary heavy and also do the Archive quest on Heroic. The next step is to turn in the reactivated core at the Gunsmith.
The gunsmith gives you another quest and after that you get the weapon. That’s all.