Most countries of ww2 made use of a heavy machine gun above rifle caliber (usually 12,7-13,2mm) or a light automatic cannon (usually 20mm) as vehicle and airplane armanent or as AA and support weapon. Issuing such weapons to frontline troops however was pretty rare.

The French 13,2mm Hotchkiss mle 1930 was used as an AA weapon on French ships and by the French air force as a close defence AA weapon at their airfield. It was not issued to frontline troops. Heavy machine guns used by the infantry were instead tripod-mounted aircooled 8mm Hotchkiss mle 1914.

The Soviet 12,7mm DShK-38 could be used as a frontline weapon and was mounted, like the 7,62mm Maxim 1910, on a wheeled trolley with a steel shield protecting the gunner, but was far more common in AA role. Production lagged behind requirements throughout the war, and lend-lease US .50 M2 Brownings had to make up a lot of the slack. As far as I know, the Soviets rarely if ever issued the DShK-38 or their lend-lease .50 M2 Brownings to frontline troops - instead it was used as an AA gun mounted on trucks - and as such it could be used as a support weapon. The primary Soviet heavy machine gun remained the watercooled 7,62mm Maxim 1910 mounted on a wheeled trolley with a steel shield to protect the gunnner throughout the war.

The British .50 Vickers was primarily mounted on ships, although it did see service as a vehicle mounted weapon and as close range air defence on airfields in limited numbers. It was never, as far as I know, issued to infantry formations. The primary British heavy machine gun remained the .303 water-cooled Vickers on a tripod throughout the war. Britain also made use of the 15mm BESA, a variant of the Czechoslovak 15mm vz.60, mounted on light tanks.

USA issued .50 M2 Browning heavy machine guns to its frontline infantry battalions - a total number of 6 per battalion from 1944 onwards. These were located with the transport elements of the battalion (mounted on or placed in the trucks and halftracks) and originally intended for air defence. Of course these weapons could and were used in support of the infantry when possible. The standard heavy marchine gun of the US infantry battalions was the aircooled .30 Browning M1919A4 mounted on a tripod.

Germany, as Sweden, Switzerland, Denmark, Britain and Italy made use of 20mm automatic cannons instead of larger calibre machine guns for the role of close air defence of airfields, to protect the support and transport elements of their battalions from air attack and at times as a vehicle moutned weapon. Germany was apt at forming kampfgruppen where light AA would often be included to add some firepower - sometimes mounted on halftracks or trucks, sometimes towed or manhandled by its crew. Sweden was alone in issuing 20mm guns (20mm Lvpv m/40) mounted in tripods to the anti-tank platoon in the heavy weapons company of the infantry battalion.

So the bottom line is that USA was the only country that issued machine guns heavier than rifle caliber to their infantry battalion, probably related to the fact that the US with its absolutey awe-inspiriting industrial ability could motorise each and every one of its infantry battalions to carry such heavy weapons and produce enough such weapons to supply each battalion with 6 of them.

Also, Germany (like many other countries, including Britain) preferred to use 20mm automatic cannons in the roles were the US, France and the Soviet Union used heavy machine guns.

To answer this question, we need to go back to the 1900s and the genesis of the heavy machine gun. When we talk about "heavy machine guns," we're often talking about two different things: weighty water-cooled guns like the .303 Vickers guns or guns that fire large bullets like the .50 BMG-firing M2HB. German ground forces don't use either in WWII, for reasons we'll get into.

The first machine guns, like the Gatling Gun or the Nordenfelt Gun, were "heavy" in every sense of the word. Large caliber. High weight. However, with the arrival of smaller designs like the St. Etienne or the Maxim, machine guns of the early 1900s become lighter guns firing standard rifle caliber ammunition. However, even these water-cooled guns were "heavy" in the sense that they weighed a great deal and weren't especially mobile.

What we'd consider the first modern large-caliber heavy machine guns, were built, oddly enough, for "balloon-busting." On the Western Front, observation balloons gave spotters a valuable perch to call down artillery and spy on enemy movements. Shooting down these balloons became an important (and hazardous) duty for the young air services. It quickly became obvious that the Vickers and Lewis .303 machine guns used by most Allied fighters just couldn't carry a big enough incendiary bullet to reliable bust balloons. As a result, the French turned to the larger, older 11 x 59mm Gras cartridge. With a newly-designed incendiary bullet, this old round got a new lease on life in 1917. Specially-built Vickers and Hotchkiss guns were chambered 11mm Gras. The Vickers guns were especially popular and were put to good use by balloon-busters like the renegade American pilot Frank Luke and the highest-scoring balloon-buster, the Belgian Willy Coppens. Although most interwar aircraft went back to using rifle-caliber machine guns, the success of the 11mm Vickers proved a heavy aircraft machine gun was viable and useful. The Germans, simply made do with their standard lMG 08 guns firing 7.92×57mm Mauser incendiary bullets.

Now, the Germans and the Allies did have even more ambitious plans for large machine guns. In 1917-1918, the emergence of (sometimes armored) ground attack aircraft like the Junkers J.I. and the large-scale deployment of tanks left Allied and German troops in a dilemma. Their once-stalwart rifle-caliber machine guns just couldn't penetrate the armor. In mid 1917, the Germans deployed the steel-cored Spitzgeschoss mit Kern "K bullet" with their MG 08s and MG 08/15s. The round could penerate about an inch (12.7mm) of armor at 100m. However, British tanks had outpaced this innovation and the newer designs were bullet-proof against the K bullets.

In late 1917, the German armaments establishment launched a design competition for a new heavy machine gun that could fire the new 13.25 × 92 mm cartridge. They hope this new machine gun would be powerful enough to penetrate Allied tanks. The MAN company's Maxim- and MG 08-based design won the contest and was designated the Maschinengewehr 18 Tank und Flieger ("Machinegun 1918 Tank and Flyer") or MG 18 TuF. Unfortunately for the Germans, the war ended before the MG 18 could be widely-deployed. The stringent limitations put on the German arms industry post-1919 by the Versailles Treaty eventually finished off the MG 18.

MG 18 and carriage. Note the external similarities to the MG 08.

The gun was a monster, weighing nearly 82 pounds and it had to be put on a carriage that weighed almost 190 pounds. In a sense, the MG 18 was less heavy machine gun and more light automatic artillery. That wasn't a wholly new concept, either. In the late 1800s, Hiram Maxim had scaled up his Maxim gun into the 37mm "Pom-Pom" autocannon. Initially deployed on warships to kill torpedo boats, Pom-Poms were also successfully used during the Boer War as anti-artillery weapons, and during WWI as anti-aircraft guns. Keep the autocannon concept in mind, it'll come back in a minute.

Meanwhile, the Americans had realized they needed a heavy machine gun of their own. The Germans had a few of their own A7V tanks, captured Allied tanks, armored ground attack aircraft, and armored pillboxes. This armor was foiling the .30-06 and 8mm Lebel machine guns used by the U.S. Army. The Americans were even more interesting in building an effective heavy machine gun for use in aircraft. Like the Germans, the Americans simply scaled up an existing machine gun. At the urging of American Expeditionary Force commanders, the Ordnance Department began work on a heavy machine gun. Initially, they tried simply using the 11mm Gras cartridge. However, the big, unaerodymanic bullet simply couldn't reach the desired muzzle velocities.

In mid-1917, John Moses Browning turned his attention to the project. Much like the Germans, Browning, simply took his existing M1917 .30-06 machine gun and scaled it up for a newly-designed .50 inch cartridge. Further experimentation lead to the .50 Browning Machine Gun (BMG) cartridge, and the water-cooled M1921 and air-cooled M2 machine guns. The U.S. Army, Navy, and Air Corps/Air Forces would make excellent use of this weapon during the inter-war years and WWWII. The Army's mania for putting machine guns on vehicles and its desire to give vehicles anti-aircraft capability mean the M2 would be mounted on jeeps, trucks, half tracks, armored cars, tanks, and tank destroyers. It was put on various AA mountings like the M16 Gun Motor Carriage, which had a quad-mounted M2s. Not to mention its deployment with infantry units as a heavy machine gun alongside the water-cooled M1917 .30-06 heavy machine gun.

The British did develop their own .50-caliber machine gun, the Vickers .50 machine gun. This fired a slightly-shorter 12.7 × 81mm cartridge than the American M2. It was also much less widely-used, only seeing service on a few armored cars, light tanks, and on ships as an anti-aircraft gun. A tripod-mounted version for infantry use was never developed or fielded. Instead, the British preferred to rely on the proven, if dated, water-cooled Vickers .303 heavy machine guns. They had excellent sustained rates of fire and were extremely reliable guns - this made them perfect for the indirect fire "barrage" tactics the British had perfected in WWI and would carry into WWII (the Americans also used the air-cooled M1919 .30-06 and the water-cooled M1917 .30-06 in similar ways during WWII).

The Germans, meanwhile, decided to take a different route. Much like the Americans, they wanted a weapons that could be used for anti-aircraft work and take out ground targets. The Luftwaffe does develop heavy machine guns like the 13mm MG 131, which fires exploding bullets, making it a kind of very small autocannon. The German army took a different route.

Instead of simply making a larger machine gun, the German army opted for a more bifurcated solution to meet the need for a heavy machine gun. The German did have a "heavy" machine gun solution. They didn't make a .50-caliber gun or use many water-cooled guns (leaving a side a few surplus MG 08s used at the start of the war. Rather, they mounted MG 34 (and later MG 42) machine guns on the excellent Lafette tripod. This allowed these guns to provide accurate fire at +1000m ranges. Although it wasn't water-cooled like the Maxim M1910, the Vickers, or M1917 heavy machine guns, the quick-change barrels of the MG 34 and MG 42 gave them similar sustained fire capability. So the Germans had a solution that 1) didn't require a new gun or new ammo to be produced and supplied, 2) achieved the same results as many Allied heavy machine guns.

An MG 34 mounted to a Lafette tripod

For anti-aircraft work, the Germans went with the 2 cm Flak 30 and Flak 38 autocannons. Although it was much heavier and bulkier than something like an M2HB, the Flak 30 and 38 fired exploding 20mm shells that packed a bigger punch than a large bullet. Initially meant only for anti-aircraft work, the 20mm flak cannons were soon pressed into service against ground targets. An autocannon spitting out 120 exploding shells a minute had understandably devastating effects on ground targets. The quad 20mm Flak 38 mounted of the Flakvierling 38 system could fire eight 20mm rounds a second out to ranges of more than 5,000 yards!

By Operation Barbarossa in June-July of 1941, some German units were attaching two 88mm and three 20mm flak guns to infantry regiments to boost their firepower. This was not done without a certain degree of trepidation, writes Marvin Knorr, Jr:

The Germans faced a difficult dilemma by placing the Flak cannons in an antitank/antipersonnel role; the greater the success of Flak against tanks and infantry, the less it would be available to carry out the primary mission of shooting down enemy aircraft. The Commanding General in a directive of July 1941, summed up (unwittingly) the German situation by admonishing the division commanders for excessive casualties among Flak crews that had been pushed to the front in various crises against tanks and infantry, and warning them to save the Flak units for air defense. In a flexible turnabout in the last paragraph, however, he stated that tank attack, more than any other factor, could result in the destruction of the division and that every Flak unit available would be pressed into use for the defense of the division in such circumstances -- so much for priority.

By 1942-1943, 20mm and the newer 37mm flak guns were integrated into defensive lines on the Eastern Front in "flak nests." Flak crews would knock the roofs off derelict buildings, cut holes in the walls, and reinforce the floors. The 20mm flak would see similar use supporting German troops in North Africa, Infantry and Western Europe.

Given the effectiveness of 20mm and 37mm flak in the close support role, the need for a large-caliber heavy machine gun was understandably reduced.

There was a movement to gave a GPMG (general purpose machine gun) that could be a jack of all trades and reduce the complexity of fielding multiple weapons for multiple roles. MG34s were abundant, being integral to German infantry doctrine, and various mounts were designed to put them just about anywhere: vehicles, aircraft, and fixed positions, in addition to the default infantry variant.

Disarmament post Versailles did result in the reduction of available heavy machine guns, but audits were cheated by retaining guns in parts, disassembled. Nonetheless, their numbers were still insufficient, as evidenced by old MG08s in rear echelon duties and the never ceasing production of more versatile & economical MG34s & MG42s throughout WWII.

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