What was with the US Army in Planet Terror?

You know the movie Planet Terror. Theres a zombie outbreak. The army was so weird. I thought they would be wearing more then just a muscle shirt and pants. They couldnt even fight the civilians toards the end. I thought that was dumb. If that was real, the army would cream them. Also at the end, wth was with the girl with a sentry gun on her leg? I mean wtf? Where the ammo and stuff?