Once you have lined up your shot simply tap on the bullet icon located at the bottom left-hand corner of the screen, just below the zoom slider.
The Rifle weapon has some exclusive functions that other weapons don’t have. You have to zoom in with the scope before you are able to shoot. You can also hold the Shoot button to hold your breath for improved accuracy (this can only be done with weapons that have the "Hawkeye Slo-Mo" perk. You can check this on the Weapon screen). Releasing the Shoot button will then fire the Rifle.
Note that you are able to customize the control layout in the “Settings” menu which will allow you to move the buttons wherever is suitable for you.