Good observations, JimDandy. Rather than compare numbers from different years, below are stats (and sources) that are all for the end of 2010.
Total Active NICS Index Records ------- 6,442,538
Felony > 1 yr and misdemeanor > 2 yrs --- 435,022
Misdemeanors Crimes of Domestic Violence - 54,559
source: NICS Operations 2010
Federal Prisoners Incarcerated -- 209,771
State Prisoners Incarcerated -- 1,402,624
Total Prisoners Incarcerated -- 1,612,395
source: Prisoners in 2010
Violent Crimes in the above ----- 740,000
source: Prisoners in 2011
(Tables 10 and 11)
Unless state prisons (as opposed to local jails) are housing a LOT of people with > 1 year sentences for misdemeanors of violence, the nearly three-quarters of a million point-in-time prison population for violent crimes looks very funny compared to the cumulative NICS total of less than half of a million.
BTW, the NICS Index is not a record of denied firearms purchase attempts; the NICS Index is the source used to determine whether a purchase is denied.
ADDED: Illegal aliens are, by definition, prohibited persons; all ICE activity is dumped into NICS to make the numbers look impressive.