I was just watching Dragnet, and it was an episode about a lady shooting her son-in-law. The gun they found next to the body was a 38 colt but the bullet they took from the body was from a 9 mm. My question is about something Joe Friday said when he was handed a bullet from the colt. He said " 6 right, .38 colt, so what?" Then the ballistics guy hands him the bullet from the body and Friday says "Hmm..6 LEFT, 9 mm" Can anyone tell me what this "6 left", "6 right" means?