ScreenToWorldPoint not accurate

I'm making a 2D platformer with a gun mechanic, and the gun is inaccurate. I'm using ScreenToWorldPoint, but it fires close to, yet not through, the mouse point. I've fiddled with it, and only made it worse. Here's my firing code:

 if (Input.GetMouseButtonDown (0) && CanFire && onGround) {
             Instantiate (bullet);
             
             BulletPhysics obj = FindObjectOfType<BulletPhysics>();
             obj.transform.position = gameObject.transform.position;
             
             CameraMovement.following = GameObject.Find ("Bullet(Clone)");
 
             Vector3 mousePos = cam.ScreenToWorldPoint (new Vector3(Input.mousePosition.x, Input.mousePosition.y, 0));
             
             obj.gameObject.GetComponent<Rigidbody2D>().velocity =
                 new Vector2(mousePos.x - gameObject.transform.position.x,
                             mousePos.y - gameObject.transform.position.y);
 
             obj.color = GunColor;
             obj.transform.GetChild (0).GetComponent<Light>().color = GunColor;
             
             LevelInformation.bulletExists = true;
 
             return;
         }

My camera is 10 units away from the surface. I'm using a custom cursor.