Cacertian RN-MBII Naval Gun

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Royal Navy Main Battery Mark II (RN-MBII)
Guns of the HMS Azalio D'Orio being installed, 1931.
TypeNaval gun
Place of originCacertianEmpireFlag.png Cacertian Empire
Service history
In service1932 – 1948
Used byCRNEnsign.png Cacertian Royal Navy
WarsSiduri War
Production history
ManufacturerCNRC Ordnance Bureau
Produced1930 – 1937
Weight120 tons
Length20 meters
Barrel length19.3 meters

ShellSeparate-loading, case charge
Shell weightAP: 1,200 kg
HE: 800 kg
Caliber405 mm
ActionHorizontal sliding block
Elevation-5° to 45° aboard the Amika Carpio-class
TraverseUp to 360°
Rate of fire2.5 rpm
Muzzle velocity820 m/s
Effective firing range36,000 meters with 800 kg HE shell at 30° elevation
Maximum firing range38,500 meters at 45° elevation

The Royal Navy Main Battery Mark II was a Cacertian naval gun introduced in the early 1930s and used exclusively on the Amika Carpio-class battleships. They were the originally intended armament of the Andrea Doria-class until the success of the RN-MBIII guns brought about a re-analysis of the new class’ armament. The remaining examples of the RN-MBII can still be found in preserved condition mounted aboard the HMS Amika Carpio at the Fumicino Naval Warfare Musuem.


The 405mm RN-MBII guns were built-up guns designed from the ground up to be a new generation of naval artillery for the Cacertian Royal Navy meant specifically for range and accuracy. The increase in caliber was part of an artillery experiment conducted by Eugenia Davion in her Project 2-4 warship design. The most notable improvement of the gun over it’s predecessors was its ability to fire a 1,200 kg armor-piercing shell at a muzzle velocity of 820 m/s utilizing a full charge.

Each gun barrel was 20,000 mm in length with a bore length of 19,300 mm and a rifling length of 16,800 mm. The guns maximum range was accomplished at an elevation of 45°; 36,000 meters with a heavy armor-piercing shell and 38,500 meters for a lighter high explosive shell. Capable of turning 4° a second, each turret could train 150° port and starboard with a 180° rotation speed of 45 seconds. The guns on turret I, II, and IV were capable of depressing -5°, but due to it’s superfiring position the guns in turret III were locked to depress only to 0°.

Service History

The RN-MBII naval guns saw extensive service during the Great War as the primary armament of the two Amika Carpio-class battleships HMS Amika Carpio and HMS Azalia D’Orio.