12GA to 357 Magnum & 38SPL RIFLED Shotgun Adapter - Chamber Reducer - Stainless | eBay
Short Lane 12 Gauge/.22LR Sub-Caliber Adapters - YouTube
Could the use of 'adapters' which fit into the chamber and swapping of the bolt, extractor and other parts allow firearms to use different calibers as long as the new cartridge is