For the usage of 'money' to be a valid system there has to be an organization to hold a value to something that in reality holds no intrinsic value. So to instead use something that is in and of itself valuable would be a much more viable alternative. Thus in a civilization that does not have someone to uphold this value the ‘money’ that we use suddenly becomes no more than worthless scraps of paper. Bullets being easily carried and having a great value on their own suddenly become a much more viable alternative to other forms of currency.
jeff185's comments