Who is Janet Napolitano planning to shoot?

Can someone please tell me why the Department of Homeland Security is planning to purchase 1.6 billion rounds of ammunition, and potentially, up to 2,700 armored vehicles? My assumption is that most in the media are afraid to ask Barack Obama or Janet Napolitano. If they did ask, I wouldn’t expect a straight answer, but I would find the response and reaction to be informative.