Someone brought one to the Small Arms School at Warminster to show us and it looked like one of those (from memory now.....) .303" chambers into which you fed a .303" 'round' that held an offset .22" cartridge. Offset slightly so that the rim fire part aligned with the normal striker in the Vickers lock.

He mentioned that it used the same(?) 'loading cartridges' as a similarly converted rifle - whatever that was - although we did had one or two