Worlds FASTEST Airsoft BBs +MACH 4 - What will they destroy

We utilized an adapter that is meant to propel crossbow bolts using a powder charge at high velocities (instead of using compressed air). This was proposed to us by Joerg Sprave of the Slingshot Channel- since he could not legally make this in Germany. He and I have also wanted to do some sort of collaboration for quite some time. The barrel of this has a 6mm ID and Danny discovered an airsoft bb would fit perfectly inside. He created a bunch of .38 loads using various amounts of powder. Not TOO ba