The National Forensic Association is an intercollegiate debating organization. I doubt they have anything particularly important to say about the JFK assassination. There was a report by the National Academy of Sciences based on acoustical analysis of crime scene recordings, but it concluded that the multiple-shooter theory was not supported by the available acoustic evidence.
Perhaps you are thinking of a report by a Texas A&M professor of statistics and a retired FBI forensic scientist from 2007 who "conducted a chemical and forensic analysis of bullets reportedly derived from the same batch as those used by suspected assassin Lee Harvey Oswald." Their conclusion was that the bullet fragments aren't particularly rare and that the matching fragments could have come from three or more separate bullets, and that previous analysis based on bullet fragments "used to rule out a second assassin is fundamentally flawed."
The important thing to note here, conspiracy buffs, is that those two reports don't contradict one another. There could be a second shooter that wasn't captured by the acoustic evidence -- but likewise, matching fragments could have come from three or more separate bullets is not an equivalent statement to "matching fragments did come from three or more separate bullets."
It's also worth noting that, in fact, the report was not done by a "national association," it never made the sweeping claim that "the official story was impossible," and the report has been criticized for naive use of statistics and generally poor writing. According to critic John Fiorentino, the paper as finally presented in 2008 was revised to address his rebuttal linked above, and "by making the revisions, the authors have effectively negated their findings just as stated in [Fiorentino's] rebuttal."
There are many criticisms to be made of the Warren Commission's handling of the investigation, and I suspect that because of that there will be people arguing about this two hundred years from now. The problem is the same here as with nearly all Grand Government Conspiracy Stories, though: even if the official story (about whatever event we're talking about) is incomplete and imperfect, that doesn't ipso facto make the official story wrong in either overall scope or final conclusion. It's worthy to question authority and to be skeptical of any official story--but there is a point where skepticism becomes gullibility: someone who automatically dismisses anything The Government says is thinking no more critically than someone who automatically accepts anything The Government says, and is ultimately just as easy to manipulate.