I tried 2 of those 12 gauge Flechette on a paper target backed by cardboard. Not as impressive as they look. Some of the darts hit sideways, some barely stuck in the target. A few penetrated the target with a less than impressive pinhole.
I would use 00 buckshot instead. Nine .33 caliber holes are more confidence inspiring.
I did some reading and got converted to #1 Buck.
2.75", 12 #1 buck over, IIRC, 26 gr. WSF. Clock out around 1250 fps.
#1 is .300".
I think of it as trying to dodge 12 308 rounds at once.
Gives you a warm feeling.