I doubt it could be done with a modern plastic shell
I can be done and is done with plastic hulls. The cut is not done where the brass meets the plastic. It is cut somewhere near the middle of cushion part of the shot cup. Not more likely to leave anything in the barrel. You're just taking part of the plastic hull out with it, and there's very little resistance in a smooth bore. Works in a pinch at closer ranges, and is lots less expensive than buying slugs. There are several videos on youtube showing how it's done.