There are quite a few reasons, a combination of American culture, experience in combat both in WW1 and outside of it, the development of different small unit tactics by different nations post WW1 as a reaction to it, and the effects of the Great Depression and need to rapidly expand the military and its equipment inventory during the run up and during WW2.

American culture at the time adored the concept of the marksman with a high powered rifle, toppling enemies at range. It still persists somewhat today. It appeals to the frontier spirit, the rugged individual through skill and grit accomplishing his goal with one shot. The frontiersman who spent his time hunting the large and dangerous animals of the wild for his food being pressed into service and using those same skills against the enemy was a myth that was much beloved around that time, the epitome of the American spirit of the citizen-soldier. That's one part, anyway. This, of course, began to run counter to what the newer officers and enlisted were running into in the field. Yes, it was effective as the often held up example of the Marines at Belleau Wood shows, however it became clear that squad level machine guns and faster firing weapons for trench clearing were needed. The other facet of the American spirit is the firm belief in technological prowess and ingenuity. That'll come into play shortly.

So, the development of sub machine guns was begun by multiple nations to deal with the close quarters nature of trench clearing. Almost all of them looked at the reports of pistols proving effective and decided why not scale up a pistol and make it automatic? Thus the early iterations were borne. In order to streamline production, and avoid reinventing the wheel, the various nations used currently produced pistol ammunition. For many nations, this meant using 9x19 Parabellum aka 9mm Luger due to the initial acceptance of it as a pistol round in the 1900s. America, however, turned it down during its trials for a new pistol, instead turning to the 1911 chambered in .45. Why? Well, their experiences in the Phillipines dealing the the Islamic insurgents in Mindanao post Spanish-American War. The close quarters jungle fighting and nature of insurgent warfare showed the weakness of the .38 double action revolver used by the army at the time, especially against a fanatical attacker who was often using opiates and could ignore the pain of being shot if it wasn't immediately fatal. Granted, the test they used are primitive by today's standards given the advancements in our understanding of both medicine and terminal ballistics (what happens once the bullet hits the targe) but they used both live and dead cattle and penetration to judge the proposed ammunition and pistol designs. The US army wanted nothing less than .45 and in 1911, adopted Browning's design for a semi-automatic pistol. So when the need for a sub machine gun came around, they did what everyone else did, and looked to simply create a machine gun that fired pistol ammunition. Their pistol ammunition was .45 so their SMG was .45. It was surprisingly effective and in fact, the British were importing it as their SMG of choice until they developed the Sten in 9mm explicitly as a cheap, cost effective weapon that could use ammunition captured from the Germans. The Sten was more of a departure from regular development than other SMGs in that regard.

Now the development of a semi-automatic service rifle began much earlier than most people realize. The US army began looking into it as far back as 1901. Post WW1, and the inevitable analysis that happens after a war, the US Army concluded that the Springfield M1903 was far more powerful than was necessary for the actual combat ranges they encountered and the US Army wanted something in semi-automatic that could serve the average infantryman in the closer ranged fighting better than a bolt action so they began development and testing in 1919. Garand actually submitted multiple rifles over the course of the 20s and early 30s, and in fact the board overseeing it went with a smaller caliber version chambered in .276. However, Douglas MacArthur stepped in and was against the reduction of caliber and requested refinements on the 30-06 version that would eventually become the M1 Garand. The myth of the American marksman was still strong amongst the older generations and wouldn't be properly shaken until the development of the M16. In addition, US Army Ordnance disapproved of the detachable box magazine and wanted a fixed internal magazine instead. Their fear was the detachable magazines would increase stoppages and would be lost frequently by soldiers during engagements. So they modified the rifle again using the en bloc clip system. It then, finally, entered production in 1937, 18 years after the initial push for a semi-automatic design.

Now, for small unit tactics. I'll try to keep it brief and non-technical. In WW1, small unit tactics became of increasing importance. Almost immediately all sides realized that they needed to change their training and equipment before 1914 was even over. The massive death toll meant that often, it would be small groups of men left from a platoon holding a position with the officer long dead. But as most training focused heavily on working in (relatively) large groups, individual soldiers were at a loss at how to continue fighting, especially during assaults. Do they hold? Advance? What's the plan? So they began to heavily focus on briefing and training everyone to be able to continue the mission independently or know the conditions to fall back and where the rendezvous is. This culminated in WW1 with infiltration tactics, with the Germans calling theirs "storm troopers." This was altogether new and a dramatic departure from the pre-WW1 mindset. No longer could you simply draft someone, press a rifle in their hands, spend 6-8 weeks marching around and polishing boots, shoot at a nice calm rifle range at paper targets, and tell them to simply follow the directions of their officers with the NCOs being treated more as messengers and extra eyes and ears in the platoon. No, that wasn't enough anymore. The NCOs began to be real combat leaders, taking squads or sections and leading attacks on their own, spread out over large distances where officers could no longer control all the minute details. Instead, they practiced with mock trench assaults with barbed wire and shooting live ammunition. Officers would brief the NCOs in full, NCOs would brief their squads and rehearse maneuvers over and over again. Signal flares distributed and everyone told what the colors meant, everyone told to know the job of the man 2 ranks above and below him so when the casualties began, even a brand new private knew what was going on. Coordination between all elements tightened, machineguns were distributed from being a division level element to company level and lower as lighter machineguns came into play. More and more authority was pushed down the ranks. Where Lieutenants were the ones leading assault elements, there were corporals and sergeants. Colonels and Generals coordinating artillery fire down to captains and majors, requesting specific targets as opposed to broad sweeping barrages. Communication and coordination became key and so did tactical initiative of the small unit leaders i.e. the NCOs and platoon commanders. Each nation would develop upon the lessons and come up with different solutions to the issues of WW1.

The Germans developed "machinegun centered" squad tactics. A German squad consisted of 10 men, 1 squad leader who would eventually be issued an MP40 instead of a rifle, a machinegunner with an MG34 or MG42, an assistant machinegunner, ammo carrier and then everyone else with rifles.The squad would be be anchored mostly around where the machine gun was. It formed the "base of fire" upon which they would then fight. Riflemen in the squad were focused on guarding the immediate flanks and once needed, several would form the assault element and close under the cover fire provided by the MG and toss grenades at the suppressed enemy. British tactics used sections instead and was somewhat similar. A section consisted usually of 8 men but was increased during WW2 to ten, the section leader was given either a Sten or Thompson and the section had 1 Bren LMG issued to it. Once in a fire fight, it would split into two groups: an assault element and the Bren group. The Bren group would suppress the enemy and the assault element would close with and destroy the enemy. They would also advance on the enemy by bounding, one group firing while the other advanced to a new position who would then begin firing for the other group to advance. Cont.

I’d like to address the relationship between the M1 rifle and BAR and their place in the rifle squad prior to and during the war. World War I had taught armies that the machine gun governed mobility on the battlefield- if you were under attack it pinned the attackers down, where artillery and mortar fire destroyed them, while in the offensive role it seized local fire superiority, suppressing the defenders and securing the ability to maneuver. It was obvious to most major powers that they had to push an automatic weapon capable of sustained and effective fire as far down the ladder as possible, and the result was that during the Interwar period most major powers modified their tables of organization, tactics, and doctrine to include a rifle squad that was built around a bonafide machine gun. The Germans famously adopted the MG34 and then MG42, and developed squad tactics that saw the rifle squad moving as a single entity. In this fashion German machine gunners displaced constantly and drove the attack forward with their fire. The British adopted the Bren gun- though it was magazine-fed it was purpose-built to be portable, and it’s quick changing barrels allowed it to deliver the sustained and effective fire expected of a squad automatic weapon. The British split their squad into two sections, the Bren and Rifle sections, with the Bren section constituting the base of fire and the Rifle section constituting the maneuver element.

The US Army did none of this during the Interwar Period. In a rather shocking turn of events considering the harsh reality the British had faced in 1914, the US Army of the Interwar Years remained steadfastly loyal to the “cult of the rifle.” Despite the lessons of the First World War the Army refused to accept that the machine gun or light machine gun was a necessary element of the rifle squad; instead it intensified its efforts to obtain a semiautomatic rifle, something it had been pursuing since the turn of the century. The Army envisioned a rifle squad that had the best of several worlds, be it accuracy, firepower, or mobility. It sought to fully equip the rifle squad with a semiautomatic rifle in order to free it from the chains of the heavy, cumbersome machine gun, but while the resulting M1 rifle was an exemplary weapon it was not capable of fulfilling that role, because, frankly, it’s not a machine gun. Thus the BAR reentered the arena.

The BAR in itself was an effective weapon. It was heavy and a had small magazine relative to its rate of fire, but it was accurate and packed a wallop. In the hands of an experienced or savvy gunner it was really a formidable weapon, as many battlefield accounts testify, but it was not suitable for the role it was thrust into. Three variants of the BAR saw action during the war, and the first two (M1918 and M1918A1) were declared obsolete by 1940. Both of these suffered from problems with overheating and the original was unbalanced. The modernized M1918A2 fixed cooling issues to the poInt of improving the effective rate of fire from 40 to 60 rounds per minute, but this was still insufficient for the role of squad automatic weapon, and all three models lacked quick change barrels, making the gun incapable of delivering sustained and effective fire in the vein that the role (and realities of combat) called for.

Interestingly enough the US Army still hadn’t “figured it out” on the eve of World War II. The concept of using the ‘ M1 as the all-purpose weapon of the squad was officially implemented in changes to the tables of organization in 1939/1940, and the BAR, which had enjoyed a spot at the squad level through the Interwar Years, was removed and placed in the company armory. It was envisioned that on mobilization the rifle company would give two BAR’s to each platoon, which would then give both to a single squad, creating a rifle platoon of two through-and-through rifle squads and one BAR squad. This only lasted until 1942, after which the BAR was returned to the rifle squad. Reflecting the return of the BAR the 1942 reforms also assigned designations to the twelve members of the squad, effectively creating scout, maneuver, and base of fire elements. This was the squad that saw action through the end of World War II.

There’s a question involved in this puzzle, however, and it’s “how did it stack up?” How did the American rifle squad compare to its contemporaries? This fits into a larger debate, one that matches the US Army up against the German army and asks “who was better?” It’s safe to say that mano-a-mano, the American rifle squad did not possess the same punch that the German squad did. The sheer number of bullets that the MG34 and MG42 put out should not be dismissed, particularly in terms of their paralyzing effect. American soldiers learned the hard way that the BAR was not going to pin the enemy down the way that the MG-series guns could, and depending on conditions present on the battlefield they could be forced to rely on supporting weapons at the company, battalion, regimental, and division level to assist them in overcoming well-defended ground. Units often attempted to correct this disparity in firepower by obtaining additional BAR’s, and late in the war the Army even took steps to try and put this into official practice.

If you’re familiar with the weapons that the Army employed during World War II, and from the specifics of your post you seem to be, you might be wondering where the .30 caliber machine gun fits into this. Each rifle company carried two light (air-cooled) machine guns in its weapons platoon; each infantry battalion carried eight heavy (water-cooled) machine guns in its heavy weapons company. Both of these machine guns were functionally the same, just with different barrels, but neither could replace the BAR at the squad level for several reasons. The water-cooled variant was heavy and expensive, and while the air-cooled variant was lighter it still couldn’t be fired from a bipod, which reduced its mobility, and lacked quick change barrels, which limited its ability to deliver sustained and effective fire. Both were seen as static, not mobile, weapons. Late in the war the air-cooled M1919A6 variant was introduced, which featured a bipod and shoulder stock, but it still did not usurp the BAR. It should be stated, however, that this weapon was a direct result of wartime experience and a recognition that frontline elements required more mobile firepower than the BAR could provide.

In short, then, the US Army entered World War II with M1’s and BAR’s because it failed to correctly identify the light machine gun as an integral part of the rifle squad. It adopted the M1 in part to render reliance on a squad automatic weapon obsolete- this concept failed. It utilized the BAR in the squad automatic weapon role because it was all that it had that best fit the requirements for such a weapon.

Only a small addition. The US might have been the only army to initially field a semiautomatic rifle but they weren't the only ones to try. The French were also working on developing a semiautomatic rifle in the 1930's, but simply weren't able to get it done. Instead, they used some of the tooling they'd developed for that and made some changes just so they could quickly turn out bolt action versions in the armory at St Etienne. That was the MAS36. Which is why the whole MAS36 is a little odd, not just the backwards bolt handle.

Producing millions of guns for an army in time for waging a war is not easy. It is far easier to make do with the millions of older ones- which is why the Civil War was fought mostly with muzzle-loaders. In WWII, an advantage the US had over the other combatants was its immense manufacturing capabilities. It'd also hadn't been damaged or bled dry in WWI.