Unknown .38 springy thingy

I have a round in my collection that resembles a .38 casing. It is a little over half of the length of a .38 Special, has no headstamp, and it looks like there never was any intention for a primer pocket to be made, as there is no hole. There is a spring that I can compress with a pencil. It will chamber in a .38, and the cylinder will rotate with this chambered. Any clue?

EDIT: It actually looks like a rimfire, but is not.

A photo would be of great help, but it sounds a little like the bottom / base part of a toy cartridge (Mattel?) which shot the toy bullet by the string power.

Mattel shootin’ shell used with greenie stickem caps

that’s it. thanks guys.