Ok, how's this...
I've lengthened the barrel and added a pump reload thingy (ack I know nothing about guns except how they work and look like)
The A one is a single barrel shotgun, (1) that is where it clicks open to reload and (2) there is sometimes a shoulder holster rest thingy there
The B is a double barreled shotgun
You can have 2 main types as far as I am aware in these, It can either be the full barrel which has a greater accuracy rate, or it can have a shorn barrel (basically it's been cut in half) and while this reduces accuracy it is much more devastating in close combat.
Also you have to change the stance of the character themselves, you need one hand on the trigger and the other on the pump mechanism. This means for a long barrel shotgun you will need to alter the body's stance and the legs also, while if you use the shorn barrel usually the gun is brought to an upright position and then pumped.
Also the gun while need reloading after each round for the single barrel, while the double can fire twice before that occurs (or it can fire simultaneously)
I remember from a couple of 3d games that there were some shotguns that could hold up to 8 bullets but I can't remember any more about them (it was a long time ago), which would require a different design.
The only other advice I can think to give is the visualization of the shotgun. It usually has a wood finish on the handle, pump, and shoulder rest, however these have been known to be made out of metal or plastic as well. If you do go with metal remember that the heat from the shot will travel across quicker to the user.
Too much info?
That's basically all I know of shotguns anyway, hope it helps.