Employee was preparing to euthanize a non-performing hog. He went to the shop and retrieved a Captive Bolt Gun and returned to the pen where the hog was located. He loaded the gun and put the safety on. As he was climbing into the pen, the gun fell. When it hit the floor, the gun discharged, and the 3" projectile attached to the gun struck the caregiver in the right thumb, puncturing his thumb and breaking the bone.