28th February, 1929
PERSONAL AND CONFIDENTIAL
(Due to arrive Canberra 29.3.29)
My dear Prime Minister,
I am asked to stress the secret nature of this letter which is for the confidential information of yourself and the Minister for Defence. 
A subject was discussed at C.I.D. meetings during last year which has been kept very secret-the development of a multiple pom-pom gun for anti-aircraft use in the fleet, and the provision of a number of these guns in the course of this and next year.
The development and trial of this new weapon has been proceeding for several years, but it was only in July 1928 that the Admiralty were sufficiently sure of their ground to bring the matter to the notice of the C.I.D., with the object of getting authority for funds to proceed with the arming of battleships and battle cruisers with this anti-aircraft weapon.
The multiple pom-pom is designed to ward off attacks by torpedo aircraft and low bombers. It has eight barrels each capable of firing go rounds per minute-a total output per gun of 720 rounds per minute. It is proposed that each battleship should be provided with two such guns at a cost of 16,000 per gun, in addition to which there would be the considerable cost of ammunition.
The gun would engage torpedo aircraft during the short period of about 30 seconds, when the aircraft were at ranges of from 3,000 to 1,500 yards before the torpedo was dropped. The great concentration of fire which such a gun could bring to bear is said to give a 75% probability of crashing the attacking aeroplane.
Critics contend that they are not proven as a suitable weapon and that the trials of the experimental gun had been carried out against gliders only and not under service conditions.
However, the matter was pressed by the Admiralty and it was eventually decided (by agreement between the Treasury and the Admiralty) that 12 guns should be provided for the fleet in the period 1928-1930 at a cost of 339,000. It is hoped that delivery of the guns will begin late in 1929 and be completed a year later.
The guns are being made by Vickers. There is evidence that certain foreign countries are aware of the development, but His Majesty's Government have instructed Vickers not to construct any of the guns for any other Government.
I have been in possession of this information for some time, but have been particularly asked not to transmit it. In this last week I have pressed the matter with high authorities at the Admiralty and have their permission to transmit to you the above letter, which I first submitted to them in draft. They hope to keep the development secret as long as possible, but will probably make public some brief statement when the gun has been developed to a more complete stage.
I am, Yours sincerely, R.G. CASEY