I wouldn't be surprised if it was released decades ago, under some name we can't identify. Remember all those case sizing lubes which are actually STP oil treatment. It might be the polyester resin used for fibreglass, which is surprisingly brittle without the glass fibres. It might also be possible to roll or spray coat the shot with some kind of wax or lacquer, and form a mass of shot, adhering but with airspaces, in a mould lined with Teflon. Aerosol paints are easy enough to crack or chip when you want them to stay intact. Now that would really be casting for shotguns!
Brittleness is important. In childhood I remember seeing shotgun cartridges into which some member of my family had poured candlewax, in an effort to make them suitable for big game, to wit German parachutists. There is a long tradition of using similar substances to concentrate the shot. But I remember hearing of experiments, I believe by Sir Gerald Burrard, in which he tried every imaginable substance, and found there was none that wouldn't either be totally ineffective, or risk dispatching the shot in a single massive lump, likely to miss any parachutist, but in the driven-game shooting he mostly knew, capable of abolishing a beater at the distance they normally stopped.
Greener, though, illustrates an early shot concentrator which was made of copper, and probably ruinously expensive, but could be done more cheaply in plastic. The shot was in two half-oval shells, pinned together through holes at their ends by a single long wire, which was fixed to a solid wad, like a giant thumb-tack. It left the muzzle pinned together, but as air resistance caught the wad, it pulled out the wire, allowing the shells to separate. Unlike solid wax, it seems bound to happen. The benefit wasn't simply in delaying the dispersal of the shot, but in delaying it until the muzzle blast was left behind, and unable to "blow" the pattern.
I've never even seen a turkey that didn't have an invitation for a Christmas party, but I do know they aren't armour plated, but prefer distant acquaintance. That article on turkey hunting with a riot gun doesn't surprise me. A short barrel, within those limits, makes almost no difference to terminal pressure or velocity. It is under about 15 inches that a barrel is liable to spread the shot wildly. The people who invented the short 4ga blunderbuss knew what they were doing. With a lot of firearms the ability to fire a second aimed shot very quickly is worth more than power. But why what's a little dementia pugilistica when you have one shot and one highwayman?