In order to change the fire rate of the weapon, you will have to aim down your sights first. When you are aimed down the sight you can switch your fire rate on console with the down button on the d-pad, and V on PC by default. You can make most assault rifles a single shot rifle this way, making it alot easier to handle the gun and land shots.
Single shot for far shots, full auto for closer shots is the way to do it. Be sure to check your pistols as well, some of those can go full auto or even burst mode. Test the gun before you go into actual combat as well.