title: Unity ignore collisions on the same layer
body: <p>I want to use Physics.IgnoreCollision to avoid bullets hitting themselves.
  The bullets are spawned on layer 8. Why is this not working? How can you ignore collisions with everything on the same layer?</p>
  
  <pre><code>// bulletscript.cs
  gameObject.layer = 8;
  
  // maingamescript.cs
  Physics.IgnoreCollision(8,8);
  </code></pre>
  
  <p>There may also be other objects on layer 8 that should also be ignored.
  (for example, the player ship).</p>