Unfortunately, Order 227 -- "Not One Step Back" -- has more than a few issues as depicted in Enemy at the Gates.
First, it has nothing to do with the topic at hand (we were talking about shooting troops who refuse to walk into minefields; something absurd in the context of Stalingrad). This matters because this crazy conflation of "everything Soviet" in WW2 makes honest discussion impossible. It's like people have a hodgepodge of random ideas, mostly taken from movies and videogames, and when you object to one they fall back to another misconception that is only tangentially related.
Second, blocking detachments weren't used in this way at Stalingrad, and were mostly about preventing troops from routing, not shooting them. Of course if you have detachments in the combat zone (as depicted by Enemy at the Gates) they will be shooting at Germans (and being shot at by them!), not shooting at their own possibly retreating troops. A cursory reflection on what "facts" from Enemy at the Gates should tell you the movie is bonkers: it simply cannot be true that there weren't enough guns and bullets to provision soldiers, and also that NKVD had plenty of guns and bullets to shoot at their own troops!
Order 227 was mostly about officers. Officers were considered traitors if they ordered unauthorized retreats, which in this context -- a war of extermination -- makes a lot of sense. Blocking detachments mostly stopped troops from panicking (but did not shoot them in the back while retreating, that depiction is so ridiculous it boggles the mind someone would believe it literally!) and NKVD acted like military police. Most soldiers retreating without orders were sent back to the front, possibly (but not always) to penal battalions in difficult sectors. Some were shot, as expected in times of war.
> Enemy at the Gates was of course a dramatized version of the situation.
No, it simply crosses into ridicule and falsehood. It's a completely absurd movie depicting tactics that weren't used. Or stating outright falsehoods like "more soldiers than guns", which demonstrably didn't happen at Stalingrad.
> But basically the notion of shipping in fresh meat across the Volga to replenish troops lost in the city was very real.
Yes, but that's not what's under discussion, is it? Stalingrad was a meat grinder for both Soviets and Germans.
> And running the wrong way was lethal.
If by that you mean "getting shot by NKVD and blocking detachments", that's mostly fictional. As in "it didn't happen at Stalingrad". An actual Soviet soldier was likely as good as dead when he set foot on Stalingrad, but that's because he was going to get killed by Germans. Go read about the battle instead of relying on Enemy at the Gates!
> And in terms of casualties this probably was one of the largest battles ever with extreme losses on the Russian side.
It was one of the bloodiest battles of World War 2. The Soviets (not Russians!) suffered greatly, but the Germans and their Axis allies also suffered a huge blow in terms of personnel. See what I mean? I confront you on falsehoods, you retreat to a technically true but completely unrelated random fact.