Would 38 Super or 9x23 work? I bet they make snap caps for those.
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I'd make it from a live round or number of them. Drill four holes big enough to allow powder out and fire primer with a torch. These things are something you have to have a handle on...then if the primer doesn't blow out drill it out and use a blot of epoxy for a filler. Also I've used pencil eraser if the primer holes allow...small erasers from mechanical pencils.
38 super should work and they are available. 9x23 winchester looks a bit shorter than 9x23 Largo. Will try researching a bit more before buying.
Use a old fired case with pencil eraser or a dab of cyclone in primer cup.
I like to use black gasket maker. Smear a gob on the end of the cartridge, let cure for 24 hours, then cut flush with a razor blade.
https://www.milsurps.com/images/impo.../0urQhV4-1.jpg
Each of those dummy rounds has been dry fired several hundred times.