1. Brass cases add zero strength to a shotgun barrel.

2. Maximum chamber pressure is called that because maximum pressure occurs in the chamber.

3. Steel inserts are called 'chamber adapters' or 'tubes' and allow a smaller gauge shell to be fired. A steel shell is a very bad idea because all shells must expand to conform to the chamber wall to properly seal the breech. Brass rebounds, as do metallic headed shells after firing so you can extract them.

"The price of good shotgunnery is constant practice" - Fred Kimble