BIG GUNS BARK: A United States artilleryman moves behind a 175-millimeter gun as it sends a .shell toward North Vietnam. The 147-pound rounds travel 21 miles across the demilitarized zone. Communist mortarmen have been firing back at the 12 guns of the 94th Field Artillery at Cam Carroll, near Dong Ha, about 10 miles south of the zone. Importance of the guns is seen in the fact that the reds have fired more than 1,650 rounds to try and silence them.