Craft with a shotgun shell, scrap metal, screws at a drill press. Requires metalworking and perhaps reloading, or can be read from a magazine.
You’d need a T-square or calipers to craft it. It wouldn’t do any meaningful damage to zombies (maybe make a crawler or cause a “Lodged Bullet” injury on your foot) as it’s not in a barrel, but it would make a nifty tripwire alarm for the outside of your base. The tradeoff here is that it’s a shotgun shell going off so will inevitably attract more zombies, but if you’re late-game you’d know what the random gunfire is.
Might be useful for clearing populated areas to divert crowds. Just an idea though. They make super advanced ones nowadays but I thought this idea worked with the time period and circumstances.