The firing position which plagued the Marines the most was located to the southwest of the hill in a U-shaped draw known as "the Horseshoe." There were at least two NVA 120mm mortars in this area which, in spite of an avalanche of American bombs and artillery shells, were either never knocked out or were frequently replaced. These tubes were registered on the hill and harassed Company I constantly. Anyone caught above ground when one of the 120s crashed into the perimeter was almost certain to become a casualty because the explosion produced an extremely large fragmentation pattern. Captain Dabney figured that it took one layer of runway matting, eight of sandbags, and one of either rocks or 105mm casings to prevent penetration of a 120mm with a quick fuze--nothing the Marines had on 881S could stop a round with a delayed fuze. Because of the shape of the hill, the summit was the only defendable terrain and thus provided the enemy with a compact target; this often resulted in multiple casualties when the big rounds landed within the perimeter. The only thing that the Marines had going for them was that they could frequently spot a tell-tale flash of an artillery piece or hear the "thunk" when a mortar round left the tube but the heavy shells took their toll. On Hill 881S alone, 40 Marines were killed throughout the siege and over 150 were wounded at least once.(88) Considering the sheer weight of the bombardment, enemy shells caused a relatively small number of fatalities at the base. Besides the solid fortifications, there were two factors which kept casualties to a minimum. The first was the flak jacket--a specially designed nylon vest reinforced with overlapping fiberglass plates. The jacket would not stop a high-velocity bullet but it did protect a man's torso and most vital organs against shell fragments. The bulky vest was not particularly popular in hot weather when the Marines were on patrol but in a static, defensive position the jacket was ideal. The second factor was the high quality of leadership at platoon and company level. Junior officers and staff noncommissioned officers (NCOs) constantly moved up and down the lines to supervise the younger, inexperienced Marines, many of whom had only recently arrived in Vietnam. The veteran staff NCOs, long known as the "backbone of the Corps," knew from experience that troops had to be kept busy. A man who was left to ponder his problems often developed a fatalistic attitude that could increase his reaction time and decrease his life time. The crusty NCOs did not put much stock in the old cliche: "If a round has your name on it, there's nothing you can do." Consequently, the Marines worked; they dug trenches, filled sandbags, ran for cover, and returned to fill more sandbags. Morale remained high and casualties, under the circumstances, were surprisingly low.(89)