You must lower the elevation angle. The greatest distance is reached with a 45 degrees elevation, but the ball takes a lot to fall. You can experiment with 30, 20, 15 degrees - but the lower the angle, the higher the force you must apply for the cannon ball to reach the target. It may be easier to set the cannon ball velocity instead of the force applied.
The function BallisticVel in the script below calculates the velocity vector necessary to reach the object target when shooting at angle elevation. It assumes the target and the cannon are at the same height, but provides some correction for small differences (small when compared to the distance):

function BallisticVel(target: Transform, angle: float): Vector3 {
	var dir = target.position - transform.position;  // get target direction
	var h = dir.y;  // get height difference
	dir.y = 0;  // retain only the horizontal direction
	var dist = dir.magnitude ;  // get horizontal distance
	var a = angle * Mathf.Deg2Rad;  // convert angle to radians
	dir.y = dist * Mathf.Tan(a);  // set dir to the elevation angle
	dist += h / Mathf.Tan(a);  // correct for small height differences
	// calculate the velocity magnitude
	var vel = Mathf.Sqrt(dist * Physics.gravity.magnitude / Mathf.Sin(2 * a));
	return vel * dir.normalized;
}

var myTarget: Transform;  // drag the target here
var cannonball: GameObject;  // drag the cannonball prefab here
var shootAngle: float = 30;  // elevation angle

function Update(){
	if (Input.GetKeyDown("b")){  // press b to shoot
		var ball: GameObject = Instantiate(cannonball, transform.position, Quaternion.identity);
		ball.rigidbody.velocity = BallisticVel(myTarget, shootAngle);
		Destroy(ball, 10);
	}
}

To test it, create an empty object about 2 units above the ground and attach this script. Make sure the cannonball don't touch any collider when instantiated, or it will not reach the target. Drag the target and the cannonball prefab to the corresponding variables and press b to shoot.

Hey guys I stumbled over this nice thread - which covers exactly what I need, unfortunately everybody is coding in unity script... i tried to translate the code to c# , but I must have made a mistake.

this is my translation:

public Vector3 BallisticVel(Transform target, float angle){ Vector3 dir = target.position - mySelf.transform.position; float h = dir.y; dir.y =0f; float dist = dir.magnitude; float a = angle * Mathf.Deg2Rad; dir.y = dist * Mathf.Tan(a); dist += h / Mathf.Tan(a); float vel = Mathf.Sqrt(dist*Physics.gravity.magnitude / Mathf.Sin(2 * a)); return vel*dir.normalized; }

here I "call" the function :

GameObject ball = Instantiate(ArrowObject,_ArrowObjectPos.position,Quaternion.identity)as GameObject; ball.rigidbody.velocity = BallisticVel(enemyObject, shootangle);

mySelf is the gameobject the script is attached to ;)

and this is the errormessage I get :

            rigidbody.velocity assign attempt for 'Sphere(Clone)' is not valid. Input position is { NaN, NaN, NaN }.
            UnityEngine.Rigidbody:set_velocity(Vector3)

I am terrible with all those Mathf. stuff. I was able to use the script like this :

public Vector3 BallisticVel(Transform target){ Vector3 dir = target.position - mySelf.transform.position; float h = dir.y; dir.y =0; float dist = dir.magnitude; dir.y = dist; dist += h; float vel = Mathf.Sqrt(dist*Physics.gravity.magnitude); return vel*dir.normalized; }

but like this - i shoot the cannon way too slow and a bit too far. Any suggestions?

You can add a script to the cannon ball prefabs that adds a constant downward force. By adjusting the amount of the downward force, you should be able to adjust the arc that the cannon ball makes.