Okay, this can be fixed in game (kinda), but it is still very buggy.
like the mosin and revolver, you must (not sure if its intentional or not) manually reload your shotgun from the inventory.
However, if you first load in two shells into the gun, you can use the action button (middle click [MC]) to reload the shotgun using the snap loader, after this is done you use MC again to reload NORMAL SHELLS...
so ya, it's as if the mosin and revolver SHOULD be able to be reloaded using MC, it's just really buggy right now. And the shotgun just needs to be sent into a cycle to get it to work.