#9 Shot? Don't you have to penetrate the brain to kill a zombie? With that short barrel, you'd need to have it at point blank, pre spread range to be effective and that's just too dangerous. IMO - You'd be better off with a mid-length field barrel and 00 buck. At least you'd be effective out to 40 yards.

My fun answer would be a Saiga 12 w/ extended mags or 20 rd. drums for heavy mid range work (I know a guy). Also a heavy chopping blade like a weighted sugar cane machete or a nicely balanced Viking style broad sword. Possibly even a Halberd with the pointy stabby bit.

BUT....if all you have to do is penetrate the brain pan, a 10/22 would actually be just about the perfect medicine. Light and pretty quiet; easy to fire for the entire family; can carry tons of rounds and preloaded mags, readily available ammo; minimal muzzle flash, recoil & rise. The lower energy 22lr gets awfully bouncy inside a skull and penetrates way less than say a 5.56.

Also, this question entirely depends on if these are the slow or fast kind of zombies. Slow zombies ala Walking Dead are lame beginner level zombies best used for training weak women and little kids. Fast zombies ala World War Z or 28 Days Later would be a real problem IMO.

I always thought a big John Deere 700 class combine would be a lot of fun too. A drive down main street would probably get a bit messy.

I need to get laid.