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
Chaszel, Author at Chaszel
Gun Adapters
Energetic Armament VOX S 7.62 w/ Direct Thread Adapter 5/8-24
X-CALIBER Survival-Rifle Gauge Adapter System - Soldier Systems Daily