Chiappa X-Caliber 12 Gauge Rifled Adapter *Choose Caliber NZ - Shotgun Parts by Gun City
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
With Battery Tactical 12GA 20GA .223 7.62x54 7.62x39 9mm Cartridge Red Laser Bore Boresighter For Pistol Rifle Collimator - AliExpress
7.62×51mm NATO - Wikipedia
Short Lane, Shotgun Adapter, 8" Rifled, 12 Gauge to 22 LR Pathfinder Series, Fits 12 Gauge Break Open Shotguns - Aztech Armory Canada
Bullseye North | Chiappa X-Caliber 12 Gauge Break Action Shotgun Caliber Conversion/Adapter Complete Set Steel Black 970.387
7.62x39 124gr FMJ, Brass, 20 Rounds, GECO
Adjustable Bore Sight .223 7.62x39 7.62x54 30-06 308 12GA 20GA 30-30 Cartridge Red Laser Boresighter - Price history & Review | AliExpress Seller - Hunting expert Store | Alitools.io
Caliber conversion sleeve - Wikipedia
The Shooters Box case and ammunition reloading gauge.
Chamber Adapter - Don't Bother | Bushcraft USA Forums