As the dust settled after the Second World War, the Allied powers looked at one another and discovered that they did not like what they saw. Many began to think another war was just around the corner, and new weapons would be needed. It had become apparent that firearms technology was moving forward at a rapid pace. The decade from 1945 to 1955 would see many Western powers switch directly from bolt-action rifles to a novel class of weapon, the battle rifle. The Warsaw Pact nations and clients thereof would effectively bypass this stage of weapon development, but that's another story. The battle rifle as a class of weapon is unique to NATO member states and allies, their former colonies and their client states.
NATO procurement has been a mess since the first countries signed the North Atlantic Treaty. In the immediate aftermath of the Second World War, the smaller founding member states of NATO were reliant on US military aid. It was not uncommon for these countries (as well as other US-aligned nations) to be using US M1 rifles and carbines while their domestic manufacturing base was rebuilt. NATO commanders desired an alliance-wide standardization in small arms and ammunition. While they had a limited standardization on US ordnance (excepting UK and Commonwealth forces) this would only last through the decade following the war, until about 1955. Around this time, member states began to adopt a new generation of rifles, quite different from the workhorses that got them through the war.
The wartime pressure on weapons development in both Allied and Axis nations produced large amounts of useful information for designers to work with once rebuilding had been completed. The German innovations in intermediate cartridges, steel stampings for production, and novel methods of operation were especially well-suited to exploitation by the victorious Allies. The StG-44 was widely examined after the war by Allied militaries and armaments companies alike. The renowned armaments company FN Herstal began work on prototype rifles using the German 7,92mm "kurzpatrone" used in the StG-44 very early. The British pursued their own .280 caliber intermediate cartridge in a quite novel rifle design of their own. A team of German engineers would refine a late-war design in France, and perfect it in Spain. At the same time, the US Army would pursue a new rifle based on their current M1.
The US Army had been slow to adopt a self-loading rifle in the first place, but once they did it was quite well liked. The M1 rifle had been the primary service weapon through the World War and Korea. During this interval the US also fielded the M1 and M2 series of carbines and the Browning Automatic Rifle. The Army began a series of trials to recitfy percieved defects in the M1 rifle while bringing in features from other weapon systems which were desirable. In order to better accomodate fully-automatic fire a slightly smaller cartridge was needed, but they were unwilling to discard the capability to make long-range aimed shots. The .308 Winchester cartridge was derived from a modification of the .300 Savage cartridge. This new round was couple with a twenty-round detachable magazine, a new gas piston system and a fire selector, bringing together the desired features. It is worth noting that the .308 Winchester is still a full-power rifle round, not an intermediate cartridge. The new rifle would be called the M14, and would be adopted for service in 1957. The M14 saw limited service in Vietnam before being phased out.
The dominance of the United States in NATO meant they were able to pressure other countries to adopt the .308 Winchester as the new standard, the 7.62 NATO. The British experiments with a .280 cartridge were rendered moot, but this wouldn't be a problem. FN Herstal redesigned their prototype for 7.62 and created the FN FAL, which would go on to be wildly successful throughout the Western world. The FAL is a milled, all-steel rifle which accepts a 20-round magazine. "Metric" variants are capable of select-fire, while "Inch-pattern" or UK and Commonwealth variants are semi-automatic only. The FAL would be adopted in some form or other by over 90 countries. The UK and most Commonwealth nations would adopt it, along with much of Western Europe. The British L1A1 version of the FAL would serve into the Eighties, famously so in the Falklands. For a short time, the fledgeling German Bundeswehr would use it as the G1, until FN refused to grant a license for production in Germany. Luckily for them, a solution would be found easily.
The German engineers who had fled to Spain produced the CETME, a refinement of a late-war design called the StG-45. These rifles were both primarily of stamped construction and used a radical new roller-delayed blowback mechanism. The design would be finalized as NATO agreed on the new cartridge. Spain would adopt it as their main service rifle. In Germany, a few men who had worked for Mauser during the war had formed their own company in Oberndorf. Heckler und Koch would eventually be licensed to produce a slightly modified version of the CETME called the G3. The G3 would be adopted by the Bundeswehr, and would be exported to a number of other countries. The CETME and the G3 have a 60% parts compatibility rate, and both are select-fire and use 20-round magazines as standard. The G3 would see most of its combat service in the third world.
Germany's old enemy France had already chosen their own rifle, and they had done it ahead of every other NATO member. In 1949 they selected the MAS 49 rifle as the new service rifle for the French Army. Due to the early adoption, it was not chambered in the 7.62 NATO cartridge and no effort was made to convert to it later. French reservations about NATO would eventually render this moot. The MAS 49 and later MAS 49/56 were chambered in the 7.5 French rifle round. These rifles were the continuation of a very robust pre-war design. They are entirely of milled steel contruction, with a direct impingement operating system. They take a 10-round detachable magazine and are semi-automatic only. They would see limited service in Indochina and extensive service in Algeria during the wars of national liberation.
At the same time in the United States, work was proceeding on developments which showed the shape of things to come. A small subsidiary company of the Fairchild aircraft company would invent a radical new design. Armalite engineer Eugene Stoner would couple advances in manufacturing techniques and materials normally used in aircraft with an extremely innovative straight-line design to create the AR-10. This rifle used an internal gas piston system and was built primarily from aircraft aluminum, with a steel barrel and bolt group, and phenolic resin furniture. They were select-fire and took a 20-round magazine. This lightweight rifle was initially unsuccessful in the United States, with an early protoype exploding during an Army trial. Eventually Armalite would license the design to a Dutch company, Artillerie-Inrichtingen. These "Dutch" AR-10s were the only military models produced for some time, with small order going to Sudan, Nicaragua and other nations. The largest single order was by the Portuguese goverment to equip their paratroopers, then fighting in Angola. The AR-10 design would go on to be scaled down to form the AR-15.
The United States quickly realized they had made a mistake in adopting the M14, and would switch to a true assault rifle as involvement in Vietnam grew. The M16 and its 5.56mm cartridge would replace the M14. While other nations would keep their battle rifles a few decades longer, they too would adopt the 5.56 round and rifles which used it. Battle rifles were an improvement over bolt-action rifles and early self-loaders, but they were heavy, long and ill-suited to close quarters combat. They were usually uncontrollable in full-auto. Even so, as late as 1964 Japan would adopt the Howa type 64 rifle, though very little is known about this weapon due to the complete lack of exported examples.
The irony of all of this is that the Warsaw Pact had seen the merit of intermediate cartridges and assault rifles before the end of the war, designing their own 7.62x39mm cartridge in 1943. The Kalashnikov rifle was adopted in 1947, with the kinks in the design and production having been worked out by 1959. For this reason, the battle rifle was a class peculiar to the West. The institutional conservatism of the US Army is at part to blame. Battle rifles live on today as second-line weapons, marksman's rifles and in special operations.
I own a new AR-10, a MAS 49/56, and US-made clones of the FAL and G3, as well as a civilian copy of the M14. These rifles are all fun, although the heavy recoil makes them less-than-ideal for extended firing. When you fire one of these, you will know it. It's difficult to pick a favorite between them.