I don't know why Eddie would want to give Rush a untraceable gun. If there was a case in which Rush would have to shoot Moe, I think her use of the gun would lead to more problems than solutions. The only reason why she would use the gun is to murder him purely for vengeance. It would be better if she were to 'unintentionally' kill him or shoot him while defending herself. In she were to use the gun during a murder, she would have to hide the body, which she would probably get away with, but it would be messy and nerve racking to hide for the rest of her life. If I were her, I would have given the gun back to Eddie and demanded diamonds.