What is the smallest weakest bullet is it the 22 or the 9mm or something else?

The weakest (the least speed times the weight) standard cartridge is the .22 BB cap. Smaller than a .22 Short, it is a round lead ball, no powder, driven by the primer only. It has no connection to a BB gun- in this case, BB means Bulleted Breach. However, there are other very rare cartridges that are smaller and less powerful than the .22 BB cap. The smallest I have seen is a 2 mm rimfire (about .08 caliber) The entire cartridge is about the size of a broken off pencil point.