This exceptionally built gun was nicknamed, very appropriately, "The Rockbuster".

Designed and manufactured in Newcastle-upon-Tyne, by Sir W. C. Armstrong in 1870, this is the best preserved example of an early 'Supergun'.

Four were originally made and sold to the Italian Navy for mounting on their battleships. The British Government, alarmed that the important Mediterranean bases might be defenceless against long range bombardment from these weapons, commissioned two guns each for Malta and Gibraltar.

Sign at the One Hundred Ton Gun Entrance

The 100 Ton Gun was introduced into the service at about the same time as control and direction of fire systems were being modernised. It is difficult to be precise but by 1884 instruments known as depression range finders(DRF) and position finders were coming into service.

Originally, there would have been a DRF mounting post on each flank of the gun (it was necessary to maintain visibility and not to be obscured by gun smoke) and by megaphone, gun data would be called out to the layers. The discovery of and development of electricity enabled both of these instruments to be operated at a distance from the guns and, in Gibraltar, were mostly quite a way up the Rock to give a good arc of visibility. This was quite useful,keeping these delicate instruments away from gun blast, and, more importantly the smoke from the discharge. There was also the advantage that, with a sea mist, where the target might not be visible to the gun but was to the cells higher up the Rock.

Eventually, therefore, these instruments were connected by electric cable to the guns. The DRF, when laid on the bow-waterline of the target, recorded the range of the target as it moved through the sea, the PF both range and bearing. The range information could be converted into Quadrant Elevation (QE) which was set on the sights of the guns, as was the bearing. As these systems were developed it became possible for the battery commander to operate the whole system form the PF cell up the hill from where QE and bearing was transmitted to dials in the gun emplacement. One of the gun detachments read out these details to the layers and the gun was laid.

When it was ready an electric signal indicating that the gun was ready was sent by cable to the BC’s cell and the gun was electrically fired from there. During this time the changing QE and bearing were constantly called out to the layers so that the gun was always on target.