Hi,
If your projectile if following your finger when you spawn it, Is it because it is a child of the gameObject you touch and drag? or is it because you have a script that also follow touches?
Basically, when you spawn a projectile, you should have it using "translate" for your purpose, where "Space" is set to "Self" and translate it using x or z axis depending on the the shape of the projectile really.
Have you tried that already?
bye,
Jean