I normally would not jump into a thread like this, but I have to put my 2 cents worth in.
The training film was just that, a biased US Army training film.
Its purpose was to instill confidence in the American weapons, and to downplay the the German weapons.
I'm very fortunate to own seven of the eight machine guns shown in the first part of said training film: the M1917, the M1919a4, the MG-34, the MG42, the MP40 and the Thompson M1a1, and the BAR.
I wish to note here, that all thirteen full auto machine guns I own, are registered and totally legal!
The only one shown in the training film that I do not personally own is the M3 Grease Gun, but I did fire one, many years ago.
I intend to talk about the firearms only, not their use doctrine.
First the submachine guns.
Of the tree, my choice to carry into combat would be a toss-up between the MP-40 and the M3.
All three are very reliable guns, very controllable at full auto, but the Thompson is too heavy to lug around.
The MP-40 and the M3 are much lighter to carry; but the advantage has to go to the M3 due to the 45 ACP cartridge rather than the MP-40's 9mm Parabellum cartridge, as the 45 ACP is just a better man stopper.
Now the light/medium machine guns.
The M1917 is a very good medium machine gun, reliable and correctly maintained will shoot shoot tens of thousands of rounds before maintenance is required, as long as the water jacket is kept full of water. It is primarily a defensive weapon, as it is heavy and is not easy to lug around. Also, it was intended to be fired off a tripod, and was a failure when used as a bi-pod mounted weapon. It required a five man crew; the gunner who carried the gun, an assistant gunner who carried the tripod, a loader who carried the water cans, and two ammo carriers.
The M1917 from personal experience is accurate due to the steady (heavy) tripod and the slow 450 rounds per minute rate of fire. At 300 yards, I can place an entire 100 round belt of ammo, fired in one long burst, into a 3 ft by 3 ft cardboard target. But this is cheating, due to the elevation and traverse locks available on the tripod (the T&E). Once you have it sighted in, and locked down, you can fire an unlimited number of rounds through a normal house sized window at 300 yards.
I love my M1917 (really a Colt M28) and its British cousin, the Vickers; but they are both too heavy to carry out to the shooting club I belong to, thus I seldom shoot either.
The M1919a4 (and its cousin the M1919a6 shown it the film) are really nothing more than a M1917 minus the water jacket on the barrel, and in the M1919a4's case, a much lighter tripod with a T&E. The M1919a6 has a shoulder stock and a bi-pod in place of the tripod. Both are a bit faster firing than the M1917, at about 600 rounds per minute. The aircraft gun versions had lighter barrel extensions and bolt assemblies, boosting their rate of fire to about 1000 rounds per minute. GI's during WWII would "acquire" aircraft MG internal parts to make their M1919's fire faster, thus I would guess the GI's also thought faster was better in combat!
The M1919a4 from personal experience is accurate enough, but with the much lighter tripod, it does tend to jump around at full auto with long bursts. To get around this, and to improve accuracy, machine gunners normally fire in short 3, 4 or 5 round bursts, "the so called gunners trigger finger twitch".
The big draw back to the M1919 is the barrel, that is not easy to change under combat conditions. The barrel is 100% air cooled, and the M1919 has a effective rate of fire of only about 150 rounds per minute, and even at that rate of fire, you will burn/erode out the rifling in the barrel, in only 10 to 15 minutes.
Because spare barrels are expensive, I normally fire a 100 round belt in short bursts and then let the gun cool off for 10 to 15 minutes before shooting off another belt.
The M1919A4 normally had a four man crew, as there was no one needed to carry the water cans.
The M1919a6 had a three man crew, as no one was needed to carry the tripod. But the M1919a6 was not an accurate weapon, as with just a bi-pod, it jumped all over the place during long bursts, requiring it be shot with short bursts to get rounds on target.
The MG34 was the Cadillac of the light/medium machine guns, period. Very well made, very versatile, and a easy one man carry. its only drawback was that it was expensive to manufacture, as it used a lot of finely machined parts. It has about a 750 rounds per minute rate of fire, and can be equipped with a 50 round drum magazine or a 75 round saddle drum magazine. This makes the MG34 a good defensive weapon, but also a great "Walking Fire" offensive weapon. The MG34 is equipped with a bi-pod for normal operations. But there is a clever tripod call the "Lafayette Mount" that turns the MG34 into a defensive medium machine gun, just like the M1919A4. Additionally the Lafayette Mount can also be used as an anti-aircraft mount, something a normally equipped M1917 or M1919 could not do.
Yes, the MG34 was also air cooled, but the MG34 had a quick change barrel setup, and normally 3 or 4 spare barrels were with every MG34 in the field. With this setup, you could quickly fire 250 or so rounds, and the in about 15 seconds change out the hot barrel for a cool one, and give the bolt assembly a quick shot of oil; and quickly shoot another 250 rounds. By time you used all 4 or 5 barrels, the first one was cooled off enough to repeat the cycle, over and over. This gave the MG34 an effective rate of fire of about 250 rounds per minute in combat conditions, for as long as the ammo held out.
The MG34 crew was 3 or 4 men, depending if the tripod was in use or not.
The MG34 from personal experience is accurate enough from the bi-pod if fired in short bursts, and nearly as good accuracy wise as a M1917 when fired from the tripod (it also has a T&E). The MG34 is my favorite belt feed machine gun, hands down. Again because spare barrels are expensive, I normally fire my MG34 with a 100 round belt in short bursts and then let the gun cool off for 10 to 15 minutes before shooting off another belt.
The MG42 was the Ford Pickup Truck of light/medium machine guns, cheap to build, reliable and accurate enough to do the job. It was also very versatile, and was used as a light machine gun with its attached bi-pod, a as a medium machine gun when used with the Lafette Mount. The MG42 also could be equipped with the 50 or 75 round drums that the MG34 used, to make it a good walking fire weapon.
The MG42 did have a high rate of fire due to its light weight bolt assembly, at about 1100 rounds per minute. A "High Rate" bolt assembly was available for use in the anti-aircraft mode, that resulted in a 1400 to 1500 round per minute rate of fire.
It can be argued that high rate of fire was detrimental to the guns performance, but it was normally fired in short 10 to 12 round bursts to improve accuracy, and not waste ammo.
The West German Army built and used a slightly modified MG42 post WWII, called the MG1 then the MG3, that used the 308 cartridge (7.62x51 NATO), and had a buffered bolt assembly that slowed the rate of fire to about 800 rounds per minute. The Yugoslavians made a copy of the MG3 they called the M53, and the Spanish made a sub scaled version that fired the 223 cartridge. Even the US Army used the MG42 as the bases for the M60 machine gun. So, the world thought the MG42 was a excellent Machine Gun Platform.
Additionally the MG42 has a much improved quick change barrel setup, over the MG34. The barrel on the MG42 could be changed out in 5 seconds by a trained and experienced loader. The MG42 crew was 3 or 4 men, depending if the tripod was in use or not.
The MG42 from personal experience is accurate enough from the bi-pod if fired in short bursts, and nearly as good accuracy wise as a M1917 when fired from the tripod, again equipped with a T&E. I like the MG42 as a belt feed machine gun, but it takes second place to the MG34. Again because spare barrels are expensive, I normally fire my MG42 with a 100 round belt in short bursts and then let the gun cool off for 10 to 15 minutes before shooting off another belt.
Finally, the M1918 BAR was ahead of its time for WWI, which it arrived too late to have any real impact in WWI combat. All in all the BAR is adequate as a squad automatic weapon, but that is all it was. It was heavy to carry, and the ergonomics of the magazine location was an issue. With the magazine sticking out the bottom of the gun, it made getting into a really low prone position difficult. A larger capacity magazine only would make that worse. Also, 20 rounds is a small magazine for a full auto weapon.The rate of fire was about 500 rounds per minute, but the BAR did not have a changeable barrel assembly. That limited the BAR to an effective rate of fire to about 100 rounds per minute for about 5 minutes at very best.
The BAR did have some good points; as it could be fired semi-auto and when shot so, it was very accurate, like a heavy barrel target rifle. But even with the attached bi-pod, it was not accurate when fired in long bursts at full auto. Thus short bursts were required to get accuracy. The BAR was a good walking fire weapon, with its only drawback being the 20 round magazine.
The BAR crew normally 2 people, the gunner who carried the BAR, and an assistant who carried ammo.
Now I do not own a M1918 BAR, I own a Colt Monitor, the civilian version of the BAR. Mechanically it is identical to a BAR, but has a carbine length barrel, a "Cutts Compensator" at the muzzle to limit barrel climb at full auto, a lighter stock and no bi-pod.
I have fired a M1918 BAR frequently over the years, and from personal experience it is accurate enough from the bi-pod if fired in short bursts. Again because spare barrels are expensive, I normally fired just a magazine or two in short bursts and then let the gun cool off for 10 to 15 minutes before shooting off another magazine or two.
Some parting thoughts.
In a totally defensive position, by myself, I would take the M1917 or the Vickers hands down.
In a combination defensive/offensive position, by myself, I would take a Mark I Bren Gun or a Japanese T99 hands down.
Here are the reasons for what many of you will consider a insane choice.
Both the Bren Gun and The T99 are relatively light weight, are extremely reliable, have a cyclic rate of around 450/500 round per minute, and are very accurate out to 300 yards. Both have an effective rate of fire of around 120 rounds per minute. Both have changeable barrel assemblies, but the Bren Gun has the advantage of a lever to change the barrel over the T99's screw out slide. The T99 has the advantages of being a bit lighter, and having less recoil than the Bren Gun. But in my opinion and experience, it would be a toss up as to what one is better.
As to cooking off cartridges in a hot barrel.
True machine guns, with few exceptions, fire from a open bolt.
There is no cartridge in the chamber until the trigger is pulled. So unless you have a dud in the chamber that failed to extract, there is zero chance for a round to cook off in any of the above discussed machine guns.
As to overheating barrels.
That is not an issue for water cooled machine guns or machine guns with a quick change barrel capability.
Lastly, that German MG42 machine gun nest shown in the film.
First, there were only two men in the nest.
Where were the other two crew members?
The other two would gave been to the sides of the nest, giving flanking cover with their MP40's.
Then, where were the other six riflemen of the German squad?
They would have been in three two's: Two to the left, two to the right and two behind the nest, to give flanking cover and rear cover to the MG42, with their K98 rifles.
Taking out that nest would not have been as easy as the training film indicated!