Because of their greater range, the enemy's 130mm and 152mm artillery batteries were located even further to the west. These guns were cleverly concealed in two main firing positions. One was on Co Roc Mountain which was southwest of where Route 9 crossed the Laotian border; the other area was 305, so called because it was on a bearing of 305 degrees (west-northwest) from Hill 881S at a range of about 10,000 meters. While the heavy caliber artillery rounds which periodically ripped into the base were usually referred to as originating from Co Roc, 305 was the source of about 60-70 percent of this fire, probably because it was adjacent to a main supply artery. Both sites were vulnerable only to air attack and were extremely difficult to pinpoint because of the enemy's masterful job of camouflage, his cautious employment, and the extreme distance from friendly observation posts. The NVA gunners fired only a few rounds every hour so that continuous muzzle flashes did not betray their positions and, after each round, quickly scurried out to cover the guns with protective nets and screens. Some pieces, mounted on tracks, were wheeled out of caves in Co Roc Mountain, fired, and returned immediately. Though never used in as great a quantity as the rockets and mortars, these shells wreaked havoc at Khe Sanh because there was very little that they could not penetrate; even duds went about four feet into the ground.(85) The 3/26 elements on Hill 881S were a constant thorn in the enemy's side because the men on that most isolated of the Marine outposts could observe all three of the main NVA firing positions--881N, 305, and Co Roc. When rockets lifted off of 881N or the guns at Co Roc lashed out, the men of Company I could see the flashes and provided advance warning to the base. Whenever possible they directed retaliatory air strikes on the offenders.[25] Whenever the enemy artillery at 305 opened up, the muzzle flashes were hard to see because of the distance and the everpresent dust from air strikes, but the rounds made a loud rustling noise as they arched directly over 881S on the way to Khe Sanh. When the Marines heard the rounds streak overhead, they passed a warning to the base over the 3d Battalion tactical radio net, provided the net was not clogged with other traffic. The message was short and to the point: "Arty, Arty, Co Roc" or "Arty, Arty, 305."(86)