At around 38 minutes, during the interrogation, the suspect's fingers change position depending on the view.
Jet looks at the spent shell casings and says that they fired armor piercing rounds and Stabler says they were fired from a MAC-10. There is no way to tell what kind of bullet was in the shell just from a visual inspection of a shell casing, nor could you tell with any degree of accuracy what type of gun it was fired from. A microscopic examination of the markings left on the casing by the gun's action can give clues as to the type of weapon, but not just by looking with the naked eye.