Double barrel shot gun you have two shots. Get a used 12 guage Browning Auto-5 and you have five shots!
The A-5 will do the job and the humpback design aids in quick aiming in dim light. I own several.
Get a new SA Saiga 12 for $450.
They usually come with a 5rnd mag but you can buy 8 and 12 rnd mag's. You can even get a 20rnd drum mag for it.
Some serious zombie stopping power!