This is an insert that can be “reloaded” into an empty/fired 12 gauge shot shell, to turn it into a dummy round aka snap cap for the purposes of practice or function checking. You will need access to a reloading press, roll crimper or similar equipment to install these. I used the Lee Load All II, which can be found quite cheaply.

I've remixed this from another file for two reasons. The first being to include a cavity where approximately 1 ounce of shot can be glued in (I used epoxy) in order to simulate the weight and centre of gravity of regular shells. The second being an issue of fitment. The inner diameter of the area in a shell where the powder is located is often narrower than the ID of the shell itself, so I was have issues installing the original version. This updated version should fit most empty shells.
 

I recommend printing with outer brims due to the small contact area of the print. Printed without supports.