We found some airsoft bbs that are made out of aluminum which only weigh .30 gram. I have been told these are for airsoft snipers and are NO more dangerous than a plastic airsoft bb with the same weight. (makes sense!) We needed a bb that was durable enough to handle the stresses involved.
We used a 12ga. adapter that Danny modified, that chambers .357 magnum. A stainless tube was welded to that which has a 6mm ID. Danny loaded some blanks with various amounts of e3 powder to drive these very light bbs to insane velocities.