Dead Trigger 2: How to unlock and upgrade every weapon

WP – "Dead Trigger 2 gives you plenty of weapons, which is certainly a good thing, because there's plenty of zombies to kill in this game. There's literally millions of zombies, as anyone who has participated in the global zombie raids will see. However, you don't get most of your guns until you unlock them. Even when you do unlock them you'll want to increase the firepower by upgrading them."