So you need a Combat / Defensive Shot Gun…
For a semi auto, we use and strongly recommend the EXCELLENT FN SLP Semiautomatic 12ga. Shotgun. We like the 22 inch barrel over the 18 inch for the extra capacity and accuracy, especially while using slugs.
This is a good review video of the FN SLP:
For a pump actions, we use the Remington 870 Marine finished version (Very rust resident for hard field use.) & the Mossberg 590.
Remington 870 Marine review here: https://www.youtube.com/watch?v=n2fb1VIiRYg
Mossberg 590: https://www.youtube.com/watch?v=PLI0j9oOhmg