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
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12GA to 410GA Shotgun Adapter - Stainless Steel
New Info on the X-Caliber Shotgun Gauge Adapters - Soldier Systems Daily
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.
Rocky Mountain Bushcraft: REVIEW: Short Lane 20 Gauge to 410 Shotgun Adapter
12 Gauge to .410 Shotgun Adapter | Buy Chamber Adapters at Cheshire Gun Room | Cheshire Gun Room