I have a gun that is positioned on the screen manually and I need to create a script in such a way that when I shoot, the bullet will reach the point where the target is targeting, which in this case is the coordinate of the half of the screen. >
I would also like to know how I can position the gun (via script) so that the barrel is aligned with the crosshair. 
It's been a long time since I started Unity3D, but I think I can do this using the Physics.Raycast .
The solution would be to make the gun always aim at the point that is well ahead of the shooter before you fire the projectile:
RaycastHit hit;
bool bloqueado = Physics.Raycast(transform.position, transform.forward, out hit);
arma.transform.LookAt(bloqueado ? hit.point : transform.forward);
After that, you create the projectile, using the same Quaternion of rotation of the weapon and position it well on the tip of it.