flechettes are like finish nails with fins
door breaching is done with compressed powder projectiles that disengrate upon impact, usually in slug form. The idea is that it will destroy the door hardware and nothing else, IT does not always work that way and door breaches are considered lethal rounds.
Flechettes might breach a door? but I don't thing so, however being mild or hardened steel they would also rebound or reflect into other targets, not really a good idea.
Flechette are basically force projection for scatterguns.
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Regards, Scattergun Bob