Can't tap off gas in the chamber... there's a shell in there and you would be blowing a hole in it.

Autos tap gas a good bit further down the barrel.

They vary in design, some use a higher pressure point than others.

Regulation for different loads is accomplished by what amounts to a 'waste gate' of some sort. Above a certain pressure, the gas is vented.

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