 O 'Brien 's main battery consisted of four 4 in ( 100 mm ) / 50 caliber Mark 9 guns , with each gun weighing in excess of 6 @,@ 100 lb ( 2 @,@ 800 kg ) . The guns fired 33 lb ( 15 kg ) armor @-@ piercing projectiles at 2 @,@ 900 ft / s ( 880 m / s ) . At an elevation of 20 ° , the guns had a range of 15 @,@ 920 yd ( 14 @,@ 560 m ) . 
