True, there will still be much energy, but since the slug won't have any direction, much of the energy that isn't lost will be spent when the slug spins wildly. There is a solid amount of energy from the gunpowder igniting, but after 70% of it is lost the millisecond that the slug is out of its jacket, the remaining 30% will be cut down considerably as it spins like a kicked football. I doubt it would travel more than ten yards or so, and it certainly wouldn't be able to pinpont enough force on any single spot to penetrate a wet paper bag.