The Rieder rifle was designed by Cape Town resident Mr. Henry J. Rieder, who worked mainly with televisions and radios but experimented with firearms when the war broke out. From 1940 - 1941, Rieder converted 18 SMLE rifles into automatic rifles. These were first brought to the attention of the British Ordnance Board by Dr. Van der Bijl, the South African Director General of War Supplies, recommended them as a means of converting existing Lee-Enfield rifles in the possession of the British Army into light machine guns.
Rieder sent three of his automatic rifles over to the UK for assessment. They performed well but certain issues prevented them from seeing service, such as constant overheating after every 100 rounds. Rieder was presented with the original SMLE rifle that the design evolved from with the serial number M45374 in 1944. This rifle is now housed at the Military Museum at Cape Town.
The Rieder rifle was simply a SMLE No.1 Mk.III rifle with a fully-automatic conversion. The gas bled from a port which was positioned about 8 inches from the muzzle and the gas tube ran along the right side of the body. It can be removed at any time with ease, thus converting the rifle back to a bolt-action system. The gas tube added about 2.5lb to the weight of the weapon and was about 12 inches long. It could be modified to allow single-shots like a semi-automatic rifle.
A wooden pistol grip was fastened to the rear end of the rifle, and a forward grip was screwed underneath the fore-end. Both vertical and horizontal foregrips were made for the Rieder rifle. Modified Bren gun magazines were used as the main source of ammunition, although 10-round SMLE magazines could be used. The Rieder rifle fired at about 250 rounds per second and could fire about 100 rounds continuously before overheating. Converting an SMLE into a Rieder rifle cost about £2 in total.