Why create a general ad when you have opponents willing to lie outright about their positions?
Maybe NRA playing it 'safe' as they have had nObama campaign threatened to file suit against their ads AND threatened broadcasters if they played the anti-nObama ads... see nObama's anti-free speech attempt:
Obama Pressures Stations to Drop NRA Spot
http://www.cnsnews.com/public/content/article.aspx?RsrcID=36590
"“When a candidate threatens a broadcaster with government sanction over the content of an ad, it is very worrisome to the First Amendment and the guarantee of free political debate,” said Sean Parnell, president of the Center for Competitive Campaigns, an organization that focuses on money in politics."