Unforgiven - local tyrant puts up "no guns" signs, but uses his own armed thugs to enforce "teh lawr". Eastwood must take up arms after laying them down, in doing so violating prohibition against bearing, to bring justice and freedom back to town.
The Man Who Shot Liberty Valance - anti-gun Jimmy Stewart's character learns that being peaceable is next to useless without the means to defend peace
Gran Torino - "Get off my lawn."
Edit: Shooter - you have to see the ending to believe it.
Further edit: Open Range - Kevin Costner is supposed to be anti-gun, but the film has a decidedly pro-2nd amendment flavor to it.