Back to Bren guns...... Those Mk1 barrels with stepped muzzles were another great innovative idea from the UKre-designers whereby, in theory, it would strangle or choke the burning powder residue. They hadn't digested and taken on board Professor Venturi's idea fully and decided that they knew best.
The trials teams at Hythe discovered that rings of diamond hard carbon were forming inside the stepped taper at regular spaced intervals along its internal diameter and length. Not only that, but the more rounds they fired, the larger the carbon deposits and therefore.........
Is anyone really interested in this saga................. If you want me to continue with the history and physics lesson just say!