The FN FAL was in development very shortly after the conclusion of World War II, during the height of assault rifle fever. Everyone (more or less) was very interested in the concept of the sturmgewehr as put into practice by Germany, and most major militaries were experimenting with similar designs of their own. The British were no exception, and they were looking at the new Belgian rifle (which would become the FAL) chambered in both 7.92×33 and .280 British, and designed in both a traditional and bullpup layout.
If you’ve ever wondered why countries made the transition from full-power heavy cartridges to lighter ones for general use (it has nothing to do with the internet myth of wounding one man to take three out of combat), here it is in a nutshell, as described by FN:
FABRIQUE NATIONALE D’ ARMES DE GUERRE , S . A., Herstal-near-Liege (Belgium), present the particulars of their new infantry weapon which they have designed to fulfil the requirement s of modern infantry. As a result of the experience and knowledge gained on all fronts during the last war, the tactics of modern infantry have undergone some change. It is no longer considered probable that the rifle will be used except at short ranges or that the light machine gun will influence the battle at ranges greater than 600 – 1.000 yards.
In this concept, it i s possible to consider the abolition
of several of the present arms which fire a small arm cartridge and to replace them by a single modern weapon. With this aim in view F.N. have designed thi s new arm embodying many of the characteristics of the machine carbine and the self loading rifle combined with a cartridge which is more accurate and powerful than that of the machine carbine but lighter and shorter than the rifle
cartridge . This combination will give to the infantry one weapon to replace the the machine carbine, magazine rifle and self loading rifle. Fitted with a light bipod, which is easily carried in the pocket, it can act as a light machine gun in firing short bursts with adequate accuracy up to ranges of 600 yards and even longer. A platoon completely armed with this weapon can develop a rate of fire and power greater than that possessed by the present day platoon.
The saving in training time, maintenance and production
by having one weapon to replace three or four different types needs no stressing.
It was all about logistics. The concept may not have been entirely successful (light machine guns still play a major role in combat, having not been adequately replaced by assault rifles), but that was the thinking in the 50s.
We have this February 1950 FN report on the FAL available for download, including a number of photos of the prototype rifles:
(1950) Prototype FN FAL manual (English)