Either breakdown one of the Khostov's you saved from when you rolled a new character to obtain the schematic,
or go to the Plaguelands Patrol
and head to The Divide.
See this video
Go through the building at the back The Divide (the one that you originally came from at the very start of the game).
Follow the path all the way until you reach the area where you ghost found you right at the start.
You'll see a large Ketch (Fallen Ship) in the distance, run toward it, board it, kill the enemies, open the chest at the back right to discover the Schematic.
Talk to Shiro at the Iron Temple and receive the We Found a Rifle Quest.
The Quest is as follows:
Collect Pages and Parts
Find manual pages and Khvostov parts on patrol in the Plague Lands.
Find 10 Manual Pages
Find 3 Khovstov Parts
For the 10 manual pages, run the campaign and kill splicers. They will not drop in partrol.
For the 3 Khovstov Parts use the following guide:
Head to The Divide as you did in step 1.
Go to the back left and through the room that originally took you to the Devils Layer strike.
The area has been updated visually, so don't let it confuse you. It's one straight line. Keep going.
On your way you'll see a Triangle Marker appear indicating you need to pick something up.
This is WEAPON PART ONE.
Once you have this, keep going forward until you reach the area with the Fallen Tank from the Devils Layer strike.
WEAPON PART TWO is next to the main central building in this area. Again, it will be indicated by a Triangle icon.
WEAPON PART THREE: Head to The Divide the same way as before.
As if you were heading toward the Earth Patrol spawn point, head through those tunnels.
WEAPON PART THREE is in the middle of this tunnel area, again indicated by a triangle.
Once you have all the parts, talk to Shiro and accept the mission A Khvostov Rising. After completing this mission you will receive your new exotic weapon.