Originally Posted by JimDandy
how many of those folks have served their time, and been released
You could piece some information together from annual release stats from the BJS, but it would not accurately reflect the population of prohibited ex-convicts. The BJS reports note that a lot of convicts are released and later reincarcerated. One person to be convicted, imprisoned, and released several times, but that person should be reflected as one name in the NICS Index.
Courts are supposed to forward information to NICS when a person is convicted. That information is supposed to be checked against the NICS Index and linked to an existing name if already in NICS or create a new record in the Index. In THEORY, the NICS Index should contain the names of all prohibited criminals, both currently incarcerated or previously released. That is why your observation that the prison population substantially exceeds the number of NICS records is so disturbing.
As far as getting information into NICS, I did notice that Schumer's UBC bill includes a section to clarify that federal courts are required to report information to NICS, suggesting that they have not all been doing so in the past.