A problem with bullet speed on different computers.

I have a strange problem with the speed of projectiles shot by a weapon. On various operating systems and hardware configs the speed is different, however is doesn't seem to have anything to do with the configuration, for example on core i7 3770k / 8GB RAM / GF 660Ti / SSD / Windows 7 64bit the speed is identical as on core i3 4000m / 4GB RAM / Intel HD 4600 / HDD / Ubuntu 14.04.1 64bit, but on some configs the bullets are a lot slower or even the speed of the projectiles varies (every fourth shot a faster bullet is spawned). I have no idea why is this happening, but I tried some advices I've found on this forums like multiplying by Time.deltaTime. It didn't make a difference. Does anyone know how to resolve this?

I'm providing some excerpts from my code below.

 void Update () {
         if (Input.GetButtonDown("Fire")) {
             Shoot();
         }
     }
     
 void Shoot() {
         Rigidbody newBullet = Instantiate(bulletPrefab, firePoint.position, firePoint.rotation) as Rigidbody;
         newBullet.rigidbody.AddForce(Vector3.right.normalized * power * Time.deltaTime);
         
     }