When ADA Claire Kincaid (Jill Hennessy) saves Ben Stone from getting jailed when talking to the judge, her hair is significantly different from the very next scene when she is talking to the officers.
Lionel Jackson, the older African-American victim, explains to Logan and Briscoe in the hospital that he knew to duck and run for cover when he saw the shotgun due to his WWII service in the 761st Battalion at Omaha Beech on D-Day. The 761st Tank Battalion was a unit in World War II and was primarily manned by African-Americans (the U.S. Army did not desegregate until after the war) however it was not deployed for action until November 1944, five months after D-Day.
Kincaid said their ballistics expert testified that Tunney's shotgun is the one that killed the victims, and the odds of another gun being the murder weapon are a million-to-one. Their ballistics expert is either totally incompetent, or committing perjury and lying her ass off. There is no way to tie a specific shotgun to shot that has been fired, a shotgun does not have a rifled barrel, it has a smooth bore that does not leave any kind of unique or distinguishing marks on the pellets as they travel through the barrel, the way the rifled barrel of a rifle or pistol does to a bullet fired through it (and TV series majorly overplay the accuracy of those ballistic tests). Van Buren even commented on this fact at the start of the episode, she said that since the shooter was using a shotgun they wouldn't be able to rely on a ballistics match. This is one of the reasons criminals like to use shotguns, especially sawed-off shotguns, and is one of the reasons why sawed-off shotguns are strictly regulated by federal and state law; it is illegal to possess a sawed-off shotgun unless it is registered with both the ATF and either the state police or county sheriff and been issued a permit.