The accuracy of the Linear Rifle and Heavy Rocket Launcher are both heavily dependent your stability. It's the small white bar on the left of the radar.
Make sure you don't have a red STABILITY warning flashing on your dash before firing. You can regen stability faster when you're grounded and stationary. If you're being mobile, you regain stability faster by not changing directions. Dodging and jumping repeatedly also damages your stability.
Other tips are to glide rather than walk as walking can cause your aim to quake and make your shots go off-target.
For the HRL, this is less important if you use it to attack big and chunky targets on the ground from an elevated position (eg, flying). The shot will land near them and still cause massive damage from the explosion. It's much less useful against highly mobile targets that fly around and that is by design.