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
Energetic Armament VOX S 7.62 w/ Direct Thread Adapter 5/8-24
X-Caliber Shotgun Gauge Adapter System
X-CALIBER Survival-Rifle Gauge Adapter System - Soldier Systems Daily