or a shotgun that uses 9mm bullets

Any pistol has exaxtly two advantages over rifles or shotguns. They are easier to carry and easier to conceal. People think pistols are super dangerous, but they are bloody hard to shoot. I've been in a pistol shooting club for 3 years now and it takes an awful lot of training to reliably hit, say, a soccer ball at 25 meters. One that's standing still, and you have time to aim. A pump shotgun with a large mag (5-7 cartridges) is in my opinion the best home defense weapon. I don't really like pumps but they are more reliable and easier to handle than semi autos. Oh, and don't load it with buckshot. That's macho idiocy. Large 36g birdshot like US3 is way more effective.