An officer injured in a SWAT training exercise Wednesday is home from the hospital after sustaining serious injuries to his left hand.Authorities said the officer was using a 37 mm projectile launcher loaded with training rounds to aid in training in Bridgewater when the device exploded in his hands, according to a preliminary investigation.The unidentified officer was treated at the scene and transported by ambulance to Massachusetts General Hospital.Authorities said he sustained severe lacerations and permanent partial amputation of a fingertip on his left hand.