to get the Hidden KARASAWA-MK2 Part (the best rifle in the game) choose the "Stop The Surface Weapon" mission requested by Balena. At the begining of the mission, fly over the surface weapon towards the "Leaving Area" line. The Karasawa is on the ground in that area. You want this rifle, trust me. (coordinates: X -000794, Y 000000, Z 000100)