I agree with Greg. The National Archives have MANY records that are not generally available to the public, because they have never had the chance/budget to microfilm them. In these days, if the $$$ were there, these records should be digitized, not microfilmed.
What gets me, back a few years ago (before the cost was prohibitive), I had the NARA microfilm the Quartermaster records of the Kentucky Orphan Brigade for me. I asked at the time, if they kept their own copy of this microfilm, for others to use, and they said no - I had paid for it, so what they made was mine (they only made the one copy for me). So if someone else were to want these same records microfilmed, the NARA would have to do it all again from scratch. Not a good plan for 140 or so year old records!
Geoff Walden