Flechette = tiny needles, fragments, or otherwise 'cutty things' moving at high speed. It does cut through armor IRL - 'armor' being kevlar, i.e. ballistic armor which is usually pretty good at stopping a bullet - which is, let's face it, a fat cone of lead. If someone is wearing something that's mostly ballistic armor, then you're golden. If they have heavy impact plating, your penetration is a lot less - because even now, the only needles that punch through metal plates are fired from mass accelerator weapons, e.g. railguns and such.
Also, KI, remember that in Shadowrun, only shotguns give you a 'cone of death'; they're just as 'roughly singular' as a standard round, so they hit in the same place.