It is called the sniper scope. It can only be found in airdrops that are spawned in by flare guns and you can find flare guns by just looting boxes or trading with other players. Once you shoot it you only have a small chance to get it
Oh yeah the flechette is hetter daybreak I killed a guy who tried to AK or maybe that was a Skorpion shoot me abd I dommed him with a Flechette round its best used against heavy armored targets you should watch out if that shotgun nailed you good means you got live running that was a flechette coming for your head