X-CALIBER Survival-Rifle Gauge Adapter System - Soldier Systems Daily
Rifled Shotgun Adapters - Gunadapters.com
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.
Rifled Shotgun Adapter 12 Gauge to 9mm - Gunadapters.com
Gear Review: TRU-BORE 12 Gauge Chamber Adapter
The Utility of 12-Gauge Caliber Adapters, The Good, Bad, & Ugly -The Firearm Blog