I do not think anyone makes a 12 gauge loading appropriate for the task. That doesn’t mean they can’t. Shotgun loadings use really fast powder. That allows a shotgun to use a relatively thin barrel. You put a barrel with more meat forward, you might be able to use a slower powder. You would have to use all brass shells, like some of the first cartridge shotguns. A solid copper or a copper jacketed steel projectile would give you better sectional density than current lead slug designs. Slower powder will also give you a lot more wiggle room on slug design. Such a design also alleviates another issue the original author brought up, fumbling with reloads trying to single load from the top. A magazine fed weapon could simply allow a mag to be dropped, and a fresh one inserted. Magazine retention should not be an issue. If a clients life is one the line, let it fall to the ground and sacrifice it.