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
7.62×51mm NATO - Wikipedia
Chamber Adapter - Don't Bother | Bushcraft USA Forums
Caliber conversion sleeve - Wikipedia
Rifled Shotgun Adapters - Gunadapters.com
X-CALIBER Survival-Rifle Gauge Adapter System - Soldier Systems Daily
Bullseye North | Chiappa X-Caliber 12 Gauge Break Action Shotgun Caliber Conversion/Adapter Complete Set Steel Black 970.387