12mm Pinfire shot construction

Here’s an 12mm shot cartridge I was taking a look at last week. The shot cartridge is brittle paper with two thin cardboard wads. Easily slid out of the case, revealing this interesting setup inside the cartridge base. There was no powder inside the case. What is this washer?

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Most pinfire case have some sort of device / piece in the head to hold & position the cap onto the pin and thus act as an reciever / chamber. Is that what your referring to as a washer?

Yes, the flat ring at the base sitting on top of the pin and cap. I guess I’ve just never seen one like this.