|If your shotgun has an internal magazine, it might come with a limiter.|
This limits the number of shells you can load to 3.
Above is what came with my Panzer 12 Gauge EG200. It's made from cheap plastic.
It goes into the magazine of the shotgun a prevents you from loading more than 3 shells.
By the way, the Panzer has a terrible manual, though they're all pretty bad.