At the Small Arms School, we were told that before members of the public could fire several of our Vickers and Bren Guns, the guns would need to be commercially (civilian) proofed. NOT SO I told them. As did the former UKMilitary proof master from Cold Meece! Antequated or what......... I mean....., what are you proofing in a blow-back SMG? The clue is in the words 'blow-back'!