Originally Posted by Goblin

In the case of 10mm, a lighter loading was introduced for the FBI because the 10mm was uncomfortable or uncontrollable for some shooters. The 40 S&W was a shortened version that gave the same ballistics as the light 10mm, but could be chambered in 9mm-sized guns. The 10mm needed a 45 ACP-sized frame. The 40 S&W can’t reliably be fired in a semi-automatic gun because the cartridges headspace on the mouth of the case. A 40 S&W might fire, but it might move enough to keep the primer from igniting. The 40 S&W can be fired in a 10mm revolver with moon clips, which serve to headspace the cartridges.