The Case of the Long-Legged Models S1 E33

They should have called this episode The Case of The Moving Guns. Check out the spoiler info:

To review: (1) Initially, the three guns are in their rightful places. (2) Mike gives Stephanie the "holster gun". (3) Eva takes the "vault gun" from the vault, shoots Castle with it, and returns it. (3) Mike takes the "vault gun" and switches it with the "holster gun". He thinks he's just taken a possible murder weapon from her and replaced it with an innocent gun. However, from this point until her arrest, Stephanie possesses the murder weapon. (4) The next day, Junior forgets to take the "Junior gun" with him, but Margo brings it to him. (5) Stephanie shows Perry the "vault gun" and he finds that it's been fired. He gives it back to her. (6) Perry borrows the "Junior gun" from Junior and fires it, then has Stephanie accept it from Junior. (7) Stephanie, willing to sacrifice herself to protect Mike, hides the "Junior gun" but leaves the "vault gun" in plain sight. (8) The police find it and identify it as the murder weapon. (9) At some point, the police also find the "Junior gun", since it's introduced as evidence at trial. They'd know it was fired, and might be evidence of Perry's shenanigans, but it wasn't the murder weapon. (10) Presumably, they verify that the "holster gun" hadn't been fired and was where it should be, in Mike's possession.