According Fighting Shotguns, Vietnam era flechette rounds extended the kill range of a 12 Ga to about 120 yards. The darts hold energy longer and only spread to about a six foot pattern.
The extended range is why they are illegal here.
I don't believe so, 'cause...rifles.
My understanding is flechettes were deemed cruel and unusual type of ordnance to deliver to an enemy combatant. They bend, twist and deform inside the human body, making them really tough for a surgeon to find. I believe they're internationally frowned on?