The Utility of 12-Gauge Caliber Adapters, The Good, Bad, & Ugly -The Firearm Blog
X-Caliber Shotgun Gauge Adapter System
Shotgun Adapter 12 Gauge to 9mm - Gunadapters.com
Miniature calibre (caliber) adapters and conversions
Caliber conversion sleeve - Wikipedia
iunio Universal Gun Cleaning Kit, with Mat and Case, Full Brass Jags, Rods and Adapters, for All Guns, Rifle, Shotgun, Handgun, Pistol, Hunting, Shooting, All Caliber