If you want a silver effect, spray it first with light grey then from a fair distance lightly mist over a couple of times with black or transparent black, adding some sharper shadows and highlighting will also help, your finished bullet holes will take the eye away from the fact that it's not real silver and the effect will be almost the same, and if your going to clear coat afterwards, which I assume you are then the whole thing will just pop.
Don't wait for the winter to do your tank, go practise on something, I want to see it and I have no patience,