I don't really understand why you'd want the robots to be shaped like a human. Surely small wheeled vehicles with mounted weapons would be cheaper and more cost-effective for most situations.
And for the situations where legs are required, such as climbing up stairs of difficult terrain, I'd have thought that a quadruped with a mounted gun would be a better solution. It has a lower profile when moving, and doesn't require as good a sense of balance as a biped.
