I did an experiment with another fellow from a gun chat forum like this a few years ago.
We tested 12 gauge factory-loaded flechette rounds with little steel darts about 1.25" long, separated from each other in the shell with a bunch of little Styrofoam pellets.
We shot them at 50 feet and 25 yards.
At 25 yards to spread was so wide only one or two darts would hit a 4 ft. square target. They totally sucked, even at 50 ft.
They did not stabilize in flight, and they all tumbled. Most hit the target sideways or even backwards. The ones (few) that hit the thin plywood target point-first generally only penetrated 1/8 th or 1/16 of an inch. Just barely enough to stick in without falling to the ground. Even some of the ones that hit nose-first from 50 feet away still bounced off or just fell harmlessly to the ground.
You've got the flechette darts. If you can reload shotshells, then you can re-create this experiment on your own. I think you'll be very disappointed in the performance.