So simply put, shotgun weapons work just fine in campaign, but in multiplayer it won't work.
Are you using the old legacy shotguns?
Legacy weapons that used to require ammo are bugged in that regard.
Aren't MOST Legacy weapons that need any ammo raped now? All weapons worked for a while about a month back but I think they cut that ability for PvP they ONLY work in Campaign now unless they changed things yet again.