The entrance-line trenches had been backed-up by next and 3rd lines: 'help' and 'reserve' trenches. Communication trenches connected all of them together. This method was strengthened with fortifications, underground shelters and thick belts of barbed wire. Why the M2 Browning has stood the exam of time and look at what https://machguns.com/product/beretta-93r-9mm-machine-pistol/