
nitpicking
As 'The round arms after 20 to 70 m of flight', I expect 100 rounds will be needed before a first hit on a millimetric SD card core.
George Nacht:
"the projectile is slow and without much static force"
With '3.2 kg and muzzle velocity is 255 m/s', I expect more than 100kg of cards are needed to stop the projectile.