For the person who opposes gun ownership on ideological or moral grounds, this film will be an hour of nails on a chalkboard. I felt it was fairly restrained in its assertions and presentation (Ted Nugent's appearance notwithstanding), far more so than Bowling for Columbine.
That the political importance of individual, private gun ownership was not even in question at the time of the writing of the Bill of Rights is easily demonstrable, and any unclarity about the meaning and intent of the Second Amendment can be swept away in short order by reading the writings of the very people who authored both it and the state laws that preceded it.
The Second Amendment provides a balance of power between the people and their government. A portion of our contemporary society may not recognize the practical value in that, but the Founders regarded the disarming of a population as a flatly hostile and despotic act:
"I ask, Sir, what is the militia? It is the whole people. To disarm the people is the best and most effectual way to enslave them." – George Mason, co-author of the Second Amendment, Speech in the Virginia Ratifying Convention, June 14, 1778
Assaulted: Civil Rights Under Fire addresses the debate primarily from a more modern vantage point, and for that reason I do not think it will prove to be very persuasive to contemporary anti-gun people.