 The fort 's main armament were the two RML 10 inch 20 ton guns — Numbers <unk> and <unk> . They were manufactured in 1879 by <unk> Armstrong and Company and originally used the Armstrong protected barbette loading system . They were made to the 1878 pattern and supplied on traversing <unk> . The guns have a 10 @-@ inch ( 250 mm ) calibre , a range of 6 @,@ 500 yards ( 5 @,@ 900 m ) with a muzzle velocity of 1 @,@ 630 feet per second ( 497 m / s ) and are capable of penetrating 11 inches ( 28 cm ) of iron at 2 @,@ 000 yards ( 1 @,@ 800 m ) . 
