Dan Rather is, was, and always will be a damn liar. The WC never conclude Oswald fired all three shots in under 6 seconds. Posner embraced the same lie in his effort to make it seem that he was setting the record straight. The WC always allowed for the possibility that Oswald took more than 6 seconds to fire his three shots. They found that Oswald fired 3 shots missing JFK with one of those shots but never came to a conclusion as to which of the 3 shots was the miss. The determined that there was no more than 5.6 seconds between the 2 shots that hit JFK and it's only the second shot miss scenario that would require Oswald to fire all 3 shots in 5.6 seconds or less. The reality is there were about 4.9 seconds between the two hits and the missed shot came before those 2 shots. We are able to determine that due to decades of time, millions of eyeballs viewing the Z-film, and technology that wasn't available to the WC.
It will forever be a black mark on Walter Cronkite, Dan Rather, and the entire CBS News organization for having propagated this lie for all these years. Either it was a deliberate lie or extremely sloppy journalism that led them to this conclusion. All they would have had to do to know this wasn't true would be to read the conclusion summary at the end of the chapter that dealt with the 3 shots.