The stalwart No1 MkIII “Smelly” served the United Kingdom well during the First World War, but by the 1920s it was growing obsolescent. The war had revealed a number of shortcomings of the design, and in the interwar years the British developed a replacement. The main issues that the new rifle would address were:
– Better mechanical accuracy, through use of a heavier barrel
– Better practical accuracy, through use of a micrometer-adjustable aperture sight
– A more practical short spike bayonet
– More efficient manufacturability
After a brief dalliance with the No1 MkV rifle in the early 1920s, the No1 MkVI was developed, which was fundamentally the new No4 rifle, just without the name. In the early 1930s a run of about 2500 No4 MkI rifles was produced, and they would go through field trials for the next several years until being formally adopted in 1939. Production actually began in the summer of 1941 at Maltby, Fazakerley, and BSA.