The last shotgun you posted is an airsoft gun. The combination of box magazine and tube magazine doesn't make sense.
H&K CAWS and USAS-12 could be a nice replacement.
A normal Pancor Jackhammer could be an alternative as well (the steampunk shotgun is based on that one). An Armsel Striker could do aswell (although it would be better fitting as a replacement for the KSG).
A little quirky but hilarious would be a belt-fed, 12ga, automatic shotgun (much like in Metro 2033).
Or a Saiga 12, either belt-fed or with a drum magazine.