How do you increase the magazine capacity of a shotgun?

Answer 1

Pump and some autoloading shotguns of popular makes with tube magazines using threaded magazine caps can have magazine extension tubes added. Typically these are sold as a kit with tube and spring. The tube replaces the magazine cap.

Box magazine shotguns such as the Saiga just need a larger capacity magazine added.

Older shotguns using magazine plugs held in by screws usually cannot increase mag capacity.