By the end of World War One the British had adopted 4 inch and 4.7 inch guns for use as anti-aircraft weapons. The shells these guns used were relatively expensive to procure and fire and as an economy measure a small 2 pounder sub calibre gun was developed that fitted inside the barrel of these larger weapons and used their breach blocks and firing systems, but fired a much smaller and consequently cheaper round for training purposes. These rounds were made by shortening the cases of standard 2 pounder ammunition and it is an example of one of these short cases we are considering today:
This case has been shortened from 158mm to 107mm in length. The base of the cartridge reveals a wealth of information:
From this we can see that the casing started out as a standard 2 pounder casing manufactured in 1917 by Vickers Sons and Maxim (VSM). The /|\ over N mark indicates that it was produced for the Royal Navy. This cartridge was then converted for use in a “2Pr Sub-Cal-H.A. Mk II Gun” in 1929 by Royal Laboratories. Note also the serations in the rim that were applied at the same time.
These sub calibre guns were used for training throughout the interwar period and in World War II they were modified for actual combat use. A percussion fired breech was fitted and they were used on harbour defence launches and other small craft, with 250 of them being produced: