Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4116936.
9mm Luger 9x19mm Parabellum Snap Cap Dummy Training Bullet Cartridge Round w/ TPU Firing Pin Cushion
Print the cartridge/bullet in PETG or ABS, and the firing pin cushion out of TPU. Choose a nice bright ridiculous color so you know it's not a real round of ammunition. Has an internal hole for pushing the plug out with a straightened paperclip or a toothpick.
A pull of the trigger makes a nice soft thud instead of a click when using the TPU plug.
Also added a rimless version if you want the round to stay in the chamber when racking the slide.
EDIT 3/14/2020: Since the original rimless version didn't grip the barrel very well, I also uploaded a version with a ridge for an o-ring, as well as a TPU o-ring to go with it. Now it stays put until you push it back out with a pencil or something similarly small.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/eV76ymCG20H