A lot of questions here! And I've answered some of them before, which I'll reproduce below!

Barrier Troops, or Blocking Detachments (Otryadi Zagrazhdeniya/отряды заграждения) were certainly a thing during the Second World War, but while watching a film like “Enemy at the Gates” might make you think that most Soviet formations needed a literal gun in the back in order to do battle, the famous opening scene is, despite drawing bits and pieces of truth from various occurrences, neither showing what actually happened, not representative of the average engagement, insofar as we can say that there is an “average”.

So, in addressing the issue of barrier troops, I feel that there are three levels to the question, each of which I will try to answer:

• Did they exist? (Yes)

• What did they do, and did that actually include machine-gunning anyone trying to retreat? (Stop desertion, but exceptionally rarely)

• Is “Enemy at the Gates”, specifically, an accurate portrayal of them, and the supply situation of the Red Army? (No)

OK, so for starters, yes, barrier troops were very much a thing, and existed in some capacity or another for the duration of the war, and have their roots in the decades earlier with Tsarist and Civil War era fighting^1 .

During World War II, the NKVD (Security Service) operated barrier troops from very early on, and while the Red Army also made use beforehand on a localized and ad hoc basis ^2 , their establishment is most associated with Order 227, issued on July 28th, which aside from establishing a large system of penal units where a disgraced soldier could atone for desertion or cowardice, also directed for the formation of “3 to 5 well-armed defensive squads” within each Army (previously they had existed no higher than the Division level) who were directed to “shoot in place panic-mongers and cowards” in the case of panic or withdrawal^3 .

Which brings us to the second part of this question. Did they actually machine gun men for attempting to simply fall back? Yes, their directive certainly gave them that option in no uncertain terms, but actually resorting to it was not the norm. We have accounts of troops being sent into battle in just that manner, but rather than being regular Red Army units, they generally make reference to either the penal battalions set up under Order 227, or the "Peoples' Volunteer Corps"/"Narodnoe Opolcheniye" (civilian levies), barely trained non-soldiers pressed into service for last ditch delaying efforts, who in some cases lacked even enough rifles to go around and instead were armed with only grenades or Molotov cocktails^4 . Sabres, daggers, or pikes were all that armed some of the workers battalions further in the city that would have seen action had the Germans broken through^5 . Army units also had shortages, but not nearly as dire^6 .

All in all, some 135,000 Leningraders from the factories and universities who volunteered (a very loose use of the word for many of them) were sent into battle in just that sort of situation, where they suffered heavy losses, with little reason^7 , and many threw down what rifles had been available to them^8 . Similarly, in Stalingrad almost exactly a year later, civilians ‘volunteered’ by the NKVD, drawn mostly from the Barrikady Ordnance Factory, the Red October Steel Works and the Dzerzhinsky Tractor Factory workers, were thrown against the Germans in delaying actions as well, underarmed and even with Komsomol members armed with machine guns emplaced behind them^9 . But this was by far the exception.

In many cases, the barrier troops were barely functional in any capacity, as they were often the bottom of the barrel, since, to quote from Catherine Merridale’s “Ivan’s War”:

Few officers were keen to spare their best men for service in the blocking units. They had been in the field too long; they knew the value of a man who handled weapons well. So the new formations were stuffed with individuals who could not fight, including invalids, the simple-minded and – of course – officers’ special friends. Instead of aiming rifles at men’s backs, these people’s duties soon included valeting staff uniforms or cleaning the latrines^10 .

Especially if a commander was not going to resort to the exceptionally harsh measure - even by Soviet standards - of taking Order 227 to the extreme, it made little sense to waste the best troops in the role. Contrary to the popular image, commanders knew that their manpower was not endless, and by mid-1942, were unwilling to resort to use such lethal methods^11 . In cases where commanders did stock the barrier force with his best troops, their positioning to the rear was often utilized in the form of a mobile reserve^12 .

All in all, the most likely way that a soldier or officer would interact with a barrier troop was not through being cut down by a Maxim, but through arrest and drumhead court martial. Especially in the case of the NKVD detachments, they wouldn’t be set up right at the line of battle, but some ways to the rear^13 , where they would apprehend retreaters, run a quick show “trial”, execute a few to make an example, and sentence considerably more to serve time in a penal unit. One representative example, of an encounter in mid-1942, recalls:

I came to know about it [Order 227] at the village of Vesely outside of Rostov.

Our retreat was barred by a special purpose detachment [of the NKVD]. A few hundred officers of retreating units were driven to a large farm. They were escorted one at a time, into a house. Three men sat at a table. They asked us about our rank and where our personnel were. The answers were generally stereotyped:

“What’s the use of asking it if everybody is fleeing. The Germans have broken through the front. What could have possibly a platoon leader or a company commander done in that situation?”

The trial was short. The sentence was passed then and there. The accused were led behind a pigsty and shot.

When my turn came, Marshal of the Soviet Union Semion Budenny suddenly appeared in the village. The execution was suspended. We were lined up. Budenny asked us “Who wants to fight?” Everybody made a step forward^14 .

In one 24-hour period during the fighting in Stalingrad, the barrier troops behind 62nd and 64th Armies made 659 detentions, but of those only 8 were shot, and 24 arrests^15 , which while certainly unfortunate for those being punished, is not indicative of the callous slaughter of any soldier foolish enough to make a tactical withdrawal. In August and September, the period of perhaps the most precarious fighting for the Soviets, of the 45,465 detentions, 41,472 were simply returned to their units, and only 664 were shot for their cowardice, with the rest arrested for imprisonment or penal combat (In comparison to the period from October to January, which saw only 203 arrests, and 163 shot), generally in view of their division to drive home the point^16 . Mostly, those who suffered were not from the lowest ranks. The simple fact is that Order 227, and the use of barrier troops in general, simply was not primarily intended for that at all. While leaving the door open for use when needed, the main target was within the officer and commissar cadres, to encourage them to prevent, let alone to not allow, unauthorized retreats by their men^17 . To quote Stalin upon his issuance of Order 227, establishing the penal units and blocking detachments, as regards his desire to quell retreat:

[W]e can no longer tolerate commanders, commissars, and political workers whose units and formations willfully abandon their positions [...]. [C]ompany, battalion, regimental, and division commanders and associated commissars and political workers who retreat from their combat positions without orders from higher commands are enemies of the Homeland^18 .

That isn’t to say that “mass prevention” wasn’t needed at times, but, in the instances where, to stem wholesale flight and prevent its spread machine guns were employed, blocking units were as likely to shoot over the heads of the troops as they were to shoot at them^19 . And when it came to executions, they were rather rare, with, overall, less than one percent of detainees during Stalingrad, facing execution^20 .

Within only a few months of the expansion of the blocking detachments under Order 227, the Red Army realized that such a scale of implementation was not worth the cost, and Oct. 29, 1942, saw their role significantly curtailed - they would not be actually abolished until late 1944^21 , but this had no effect on the NVKD units which continued in their role as backstops^22 , nor did some Red Army commanders cease using the formations on an ad hoc basis. Whether or not the blocking units had played a part in it, inspectors did see a considerable improvement in the morale and resolve of the forces by that August ‘42^23 , as also demonstrated by the declining numbers picked up by the NKVD as mentioned earlier.

Continued below

/u/Georgy_K_Zhukov has pretty well tackled some of your questions, but Ill just try and tackle your question regarding armored support.

In the 1920s and early 30s, the Red Army co-developed armored theory with the Weimar Republic. While this development was abandoned following the rise of Nazi Germany, it left the Red Army a legacy of sophisticated concepts regarding the tactical and operational use of armor. In the early 1930s, the Russians also developed a number of tank designs, many of which replicated successful British and American designs. In many ways, the Red Army had one of the most advanced tank forces in Europe in the early 1930s. However, the latter half of the 1930s was unkind the the Red Army.

Starting in 1936, the Stalinist regime began to purge leaders of groups which Stalin perceived as threatening to his regime. In 1937, his attention turned towards the Army. Between 1937 and 1939, 3 of 5 Soviet Marshals were purged, 13/15 army commanders, 50/57 corps commanders, and over 150 of the 180 division commanders in the Red Army. Some of these commanders were executed, while others were imprisoned or forced out of Soviet political life. While some of these officers would eventually return to the Red Army, the purge completely devastated the Red Army. To replace those who were purged, many Red Army officers were promoted from a relatively low rank up to command of divisions, corps, and armies. They lacked the training, experience, and theoretical framework required to use the Red Army as it had been designed. Further, the purges had pushed the Red Army to be more conservative and orthodox. Officers were afraid that if they diverged from accepted doctrine, they would be the next on the chopping block.

All these political attacks slowed down the rate of technological and theoretical development of the Red Army. It was only in 1939 and 1940 that more advanced tanks like the T-34 and KV-1 entered into production and by 1941, few of these tanks had reached the frontline. Instead, most of the Red Army was equipped in tanks which were advanced by 1930s standards, but were obsolete against the cream of the German Army. In addition, many of the tank commanders in the Red Army did not know how to best employ the forces they did have. As a result, many tank divisions were clumsily employed in hasty and unprepared counter-attacks which devastated the lightly armored Soviet tanks.

However, by 1942 the Red Army had changed. Many of the incompetent commanders of 1941 had been replaced by better generals, or had been whipped into fighting shape by both STAVKA and the Germans. The tank forces in particular had underwent a serious transformation. The obsolete tanks of 1941 had been replaced by masses of KVs and T-34s into tank corps, which were designed to replicate the German Panzer Division. These Corps were still sometimes asked to do the impossible, to halt serious German offensives with hasty and unplanned counter-attacks. However, Operation Uranus was an excellent example of operational preparation and and tank exploitation. The Soviet tank forces performed exactly as designed, and they smashed through Romanian and German defenses. By 1942, the Red Army had revived a lot of the armored concepts it created in the 1930s, and successfully employed them against the German Army.

Sources:

David M Glantz, When Titans Clashed: How the Red Army Stopped Hitler

David M Glantz, Stalingrad Trilogy

David M Glantz, Soviet Military Operational Art: In Pursuit of Deep Battle

Robert Conquest, The Great Terror: A Reassessment