X-CALIBER Survival-Rifle Gauge Adapter System - Soldier Systems Daily
The Utility of 12-Gauge Caliber Adapters, The Good, Bad, & Ugly -The Firearm Blog
12GA to 357 Magnum & 38SPL RIFLED Shotgun Adapter - Chamber Reducer - Stainless | eBay
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.