Most games use copies of military hardware, from around the world, and some use futuristic guns also based of those designs, but what i'd like to see are some of the great non-military guns that are futuristic, but actually exist now.
Calico guns are a great example, with 100 round top loading chambers, completely waterproof (can fire underwater!), and incredibly fast reload times.
They have other unique designs elements too, might give you some ideas for stylizing the ones you make.
http://calicolightweaponsystems.com/hom ... p?page=9mm shows some examples of what i'm talking about. Because, the truth of the matter is, calicos are better than most military guns, more functional, faster reload, etc. etc. They'd be much more useful during zombie attacks than any ak-47 would be.