these are shotgun adapters aka chamber reducers aka shotgun inserts and called by a few other names. they allow you to fire lower gauge (caliber) shotgun shells out of a larger bore
Bullseye North | Chiappa X-Caliber 12 Gauge Break Action Shotgun Caliber Conversion/Adapter Complete Set Steel Black 970.387
Sterling, 7.62x39mm, 123 Grain, Steel Casing, Case of 1000 Rounds UPC: 8698779967604 - Global Ordnance