The costume itself is fairly straight forward, but to be convincing it requires two things that aren't likely to be ready at hand: a mini-gun and a bandoleer of extremely large bullets. A simple google search will turn up plenty of other people's attempts at the same thing, but they all seem to lack something... craftsmanship!
Seeing that I decided, as any good instructables junkie would, to make the best effort possible and document the process for anyone else who wanted to do the same.
All together the costume will probably cost about $75, assuming you have all the tools but none of the parts, and take about 20-25 hours to assemble. Time consuming, sure, but totally worth it.
The first step, before even looking for materials, was to spend an hour or so looking at all the best source images I could find and sketching out my design. Even if you plan to follow my design to the letter I'd recommend doing something similar, I find it helps immensely to have a really good mental picture of the problem at hand before starting.