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
45 COLT to .22 LR Caliber Adapter - Chamber Reducer - Stainless Set of 6
Caliber conversion sleeve - Wikipedia
45 COLT to .22 LR Caliber Adapter - Chamber Reducer - Stainless Set of 6
Gun Adapter
The Utility of 12-Gauge Caliber Adapters, The Good, Bad, & Ugly -The Firearm Blog
Wood Trekker: 12 Gauge Shotgun Chamber Adapters: The Good, The Bad, and The Ugly
Shoot any round through any firearm with subcaliber adapters - Carolina Sportsman
20 Gauge Shotgun Adapter – AnytimeAmmo
EZshoot Boresighter's Adapters & Screws for 0.177 to 0.54 Caliber Bore
Miniature calibre (caliber) adapters and conversions
Chiappa X-Cal 12 Ga Adapter Set of 8 Inserts Case
X-Caliber Shotgun Gauge Adapter System - Athlon Outdoors
410 to .22 LR conversion CHAMBER Adapter*** (Judge)*** 2.5 CHAMBER - $49.99 | gun.deals
45 COLT to .22 LR Caliber Adapter - Chamber Reducer - Stainless Set of 6
X CALIBER Shotgun Adapter Field Test with Tim Ralston of National Geographic 's Doomsday Preppers - YouTube
Runway Sub-Caliber Adapters for 26.5mm Flare Guns - YouTube
Eagle Gun Range -Macon GA - X-Caliber Ammo Adapter Set for Shotguns. It's an 8-piece set of drop-in adapters that measure seven inches long each, and it comes with a carrying case.
Short Lane - Finest Shotgun Adapters - GUNADAPTERS.COM
45 COLT to .22 LR Caliber Adapter - Chamber Reducer - Stainless Set of 6
Chiappa Firearms X-Caliber 4 Caliber Adapter Kit for 20 Gauge | $5.01 Off w/ Free Shipping