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.