As a Primary Source for 2002
We can use this film as a primary source for the 21st century because of its glossing over the racial tension between Amsterdam and Jimmy Spoils (the only prominent black character). This not only embodies the mentality of a 2002 audience but it is also completely false. Amsterdam, being an Irish immigrant, would probably not have overcome his racial prejudices so easily and would not likely extended friendship toward a black man. Instead of showing this, Scorsese ignored this realistic 19th century tension in the film.
As a Secondary Source for mid-19th century
Martin Scorsese does a fantastic job of portraying what life was like in the mid- 19th century. While some of his characters and themes are exaggerated, the overall picture of life being dirty, dangerous, and violent, is accurate and demonstrates a tenuous life filled with tension between classes, ethnicities and race.
 Killing of Bill the Butcher, https://www.youtube.com/watch?v=jn8SVc374U0
 Ending scene, https://www.youtube.com/watch?v=p-8Lu7MRjQs