Short Lane, Shotgun Adapter, 8" Rifled, 12 Gauge to 22 LR Pathfinder Series, Fits 12 Gauge Break Open Shotguns - Aztech Armory Canada
Barrel Shroud/ Brake Adapter Protector Vented
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
Could the use of 'adapters' which fit into the chamber and swapping of the bolt, extractor and other parts allow firearms to use different calibers as long as the new cartridge is