Not sure if they are still available, but Winchester use to make them in various calibers for pistol and rifle. They use a regular case with an empty primer pocket. The vent was not drilled. On rimfires, they driiles a small hole at the center of the base. Then they apply a chemical that gives them a dark chocolate brown color to the outside of the case. You can clearly tell that they are dummy rounds by the color. Perhaps you can do the same by bluing the cases, in addition to using spent primers. I also know that Birchwood Casey makes a Brass darkener. .....
Be Safe !!!