Gun surfboard designs are made for big waves and most surfers never surf in conditions where they really need them. Just in case you are one of the few and the brave, we'll tell you a little bit about them. They are usually longer, ranging from 6'6" all the way up to 10'. When riding a really big wave, a small board can start to wobble and so these boards come with extra thickness as well. They are narrow in the nose and tail to allow the rails full contact with the wave. You will sometimes see a gun designed with a specific wave in mind, for example; Mavericks Gun or Waimea Gun. These boards are made specifically for the unique characteristics of that wave. You need all the help you can get out there because any mistakes on a big wave will have heavy consequences.
When you are riding a gun there is one thing going through your mind: "make the drop". You will sometimes see big wave drop-ins where the surfboard is almost completely vertical as the surfer races down the face of a wave. After the drop-in the main focus is navigating the wave and handling any chop.
There are also semi-guns which are surfboards designed to function like guns but that aren't quite as big. You might see some of these come out if the waves ever get in the head and a half to double overhead range. These boards can withstand a little bit more pounding than a typical shortboard and will allow for increased stability when traveling in a straight line at high speeds.