Shotshells and rifles have a different order of magnitude of pressures in the chamber. A rifle cartridge is supposed to EXPAND and grip the sides of the chamber, then SHRINK back, so that it can be extracted. Plastic just does not do that as well as brass. Steel does better than plastic, not as well as brass. And yes, there have been experiments to use other things- like leather and cardboard (see the Marston cartridges from the mid 1800s). And FWIW, I ahve a few ALL PLASTIC shotshells in my collection- the extactor has a tendency to rip thru the plastic rim, so they did not catch on- but since they are translucent, they are kinda neat- you can see the powder, shot wad, shot cilumn, etc. Think the more common brand of these was Wando.