The gun is gone from the ME3 Missions prize list. Why? It was listed under the highest rank last I checked, why was it removed?
It looks like we probably won't be able to get our hands on any of these puppies- they sold out of 'em so fast! But, alas, if you complete Mission 6, we'll figure out some pretty sweet and potentially even more militaristic alternatives, or you may have your pick of the rest of the prize page.
In short, the replica rifles were never available as prizes, due to their limited production run. Unfortunately, SE did not realize this, and thought they'd make a good top prize. When it became apparent that they wouldn't be able to acquire them, they removed it.