At one time, I had half a case of military 12 gauge Flechette rounds, but I shot most of them up many years ago. I think I've still got a couple of them around somewhere.
Those original loads used a small flat steel slug under the flechettes to push them out of the shell and down the barrel. I would imagine they found out early on that the sharp back edges of the fins would cut through the nylon wads and either not be pushed out, or stick in the wad and not disperse. The flat steel washer was small enough to fit through the choke of the barrel, and the plastic sleeve surrounding the flechettes was pretty thick to protect the barrel.
In shooting those flechettes, I pretty much considered them worthless. They don't stabilize until they're about 10 yards from the muzzle, and then once stabilized, they make needle sized holes. If I remember correctly, there were about a dozen flechettes in each round, so there isn't a whole lot of density to the "pattern".
They could probably be loaded, but it really wouldn't be worth the effort, in my opinion.
Hope this helps.
Fred