The fort 's main armament was the two RML 10 inch 20 ton guns — Numbers 3470 and 3472 . They was manufactured in 1879 by WG Armstrong and Company and originally used the Armstrong protected barbette loading system . They was made to the 1878 pattern and supplied on traversing slides . 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 ) .