Both the 18/25 pdr and the 25pdr used 2 piece ammunition with a variable charge propellant system. The charges were set on the gun position by adding or removing propellant bags from the cartridge.
There was a High Explosive round plus smoke and star shell rounds. In addition an Armour Piercing round was also available and which came in 2 variants.
The first was a typical solid shot round of 9.1 kg and which had limited penetrative powers .
The later variant was an Armour Piercing Cap Ballistic Cap (APBC) round which by virtue of its design had far better penetrating powers. However to achieve the required performance it needed to be fired at a very high muzzle velocity which meant that only certain marks of the gun could fire it.
