The most fun I've had recently with a shotgun is with a double barreled 20 ga black powder shotgun. You can custom each load, and even have different load in each barrel. The phrase 'buck and ball' is a good example where one side is a shotgun load, with the other barrel carrying heavy buckshot.

It's addictive to shoot, and surprisingly accurate.

The small shells would work well in a pump, but I'm not sure about a semi-automatic. It would be fun to try though. Shotguns are more of a secondary firearm for me, although I've thought about a circuit judge!

Be blessed!