No indication is given about the next to fire (at least that was never implied, or explicitly mentioned), it&#x27;s about the probability.  The shaded red disks simply show which chambers might have rounds in, and which might be blank.  Rather that enumerate all possible combination of the &quot;6 choose n&quot;, I showed the canonical form of &quot;two together&quot; (of which there are 6 cases), &quot;one chamber inbetween&quot; (ditto) ... etc<p>I tried to make that clear with the phrase &quot;without loss of generality&quot;