The introductions of new rifled coastal artillery by the British in the 1880’s had increased the ability of guns to penetrate the belt armour on the new generation of warships. However, there was still concern with some warships having armour over eighteen inches thick. One answer to this was to use High Angle Fire (HAF) guns that gave plunging fire. The shells dropping down from above would attack the deck armour which was far more vulnerable. This concept had been tried in the 1860 using smoothbore mortars but had be dropped with the introduction of rifled ordinance.
After some experimentation it was decided to use converted 9-inch RML guns as the basis for the 9-inch and 10-inch HAF guns.