However, engineers should not be satisfied with hearsay! Let's take a look at some test results of the D-25T gun mounted on the Soviet IS-2 tank.
The caption for the first picture reads: "Figure #2: Graphical depiction of shooting results during a D-25 accuracy test, from a distance of 1000 m." The small circle, marking the 5 most accurate shots, is 24 cm in radius. The large circle, marking all shots, is 80 cm in radius.
The caption for the second figure reads: "Figure #3: Graphical depiction of shooting results during a D-25 accuracy test, from a distance of 1900 m." The small circle, marking the 5 most accurate shots, is marked 45 cm in radius. The large circle, marking all shots, is 120 cm in radius.
What does this all mean? The casual viewer might see that this accuracy is good, but "good" has no place in engineering. Let us compare these results to contemporary German guns, the Tiger and Tiger II (often erroneously referred to as King Tiger).
Now, let's go up to 2000m. The accuracy of German tanks decreases slightly: 87% for the Tiger and 85% for the Tiger II. To see how the IS-2 measures up, let's hold up an imaginary 2.5 meter by 2 meter target to the distribution, like the Germans would be testing against.
As you can see all shells fall within the target's surface! By German metrics, the IS-2 has 100% accuracy at this range, even higher than the accuracy of the Tiger and Tiger II. Now, this is only one 10 shell trial, and the Soviet distance is 1900 m instead of 2000 m, so I cannot readily say that the IS-2 is more accurate than either of the German Tigers, but the reader can clearly see that the accuracy of the Soviet D-25T gun is at least comparable. According to a table of penetrations that will be the topic of a later post, the D-25T's armour piercing shells could defeat 129 mm of flat armour or 105 mm of armour angled at 60 degrees, enough to penetrate all armour of German tanks, aside from the upper glacis plate of the King Tiger.
Let's take a look at another Soviet tank gun, the ML-20S howitzer that was mounted on the SU-152 and ISU-152 tank destroyers. This test uses a reduced propellant charge, the variable #6 one.
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The ML-20S was a howitzer, which are characterized by a larger caliber to barrel length ratio. This leads to a round that is not as accurate as one fired from a gun. Let's take a look at the results.
At a distance of 1000m, the ISU-152 fired 10 HE-fragmentation shots towards the target. 50% of the shells landed within a circle with a 32 cm radius, and 100% of the shells landed within 96 cm. Obviously, not quite as good as the IS-2, but still respectable, and still a 100% accuracy metric by German standards. At this distance, an AP shell penetrated 124 mm of armour, which was enough for any German tank that was not a Tiger II. However, it did not have to penetrate. The 152 mm howitzer often shot HE rounds only, which was enough to literally tear any tank unlucky enough to be hit by them apart.
In conclusion, despite what the popular history might have told you, Soviet tanks were very dangerous to German tanks, even at a large distance!
Edit: a few people were questioning why the distributions are circular, instead of oval. They're really not, which I explain here, along with data for a lot more guns.