Flechettes were tried in Vietnam, where they proved to be a failure.
The idea was to offer longer range use of the shotgun.
Most of them were nail-like aluminum needles, with tiny formed fins.
Half were loaded in the shell backward to allow fitting more in the shell.
In actual use, they were deflected or stopped by brush, and have no striking force.
Although they did carry farther in a denser mass than buckshot, they failed to penetrate.
The big problem is, they have no mass, so they fail to have any stopping power.
It's sort of like being stabbed with needles, instead of impacted by shot.
The worst failing of them is, people shot with them in the stress of combat often don't even know they've been fatally hit until they bleed to death internally.
There were too many cases of even tiny Vietnamese shot with them, who continued to attack for some minutes.
The bottom line is, these are NOT "stoppers", and although people are often killed by them, they die.........eventually.
"Eventually" is not something you want in your face in your bedroom at 3:00 am.
Bottom Line: Another good idea that just failed to pan out in the real world.