It's actually a Thompson submachine gun and an underslung Benelli 12 gauge? I forget who the shotgun manufacturer was, but for the most part, this is two real guns stuck together with fiberglass and epoxy.
You could really kill aliens with this gun. Think about it, then rate accordingly.
I don't think that .45 would be the best for killing aliens, at least not the ones from the movie with the hard exoskeletons. Aliens from neighboring countries, probably, but I'm generally opposed to that sort of treatment of immigrants.