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