They fire 400 @-@ pound ( 181 kg ) projectiles using a 130 @-@ pound ( 59 kg ) gunpowder cartridge . Though they was insufficient against contemporary battleships , they was seen as adequate for the light cruisers that the fort was expected to face . The carriages weighed 13 long tons ( 13 t ) and moved on semi @-@ circular traverses . The guns was loaded either manually — using the Woolwich ( derrick ) system — or via the Armstrong mechanical loader . The Armstrong system enabled loading of the guns from behind the protection of the rampart whereas the manual system required at least two crew to be exposed on the rampart 's top . Despite this the guns was manually loaded for most of their operational life as the Armstrong cable drive was faulty and too difficult to maintain . The guns had a 14 @-@ man crew and loading took about 2 minutes , a rate of fire that was only slightly faster using the Armstrong equipment .