Thursday 21 August 2014

D-5 and S-31 Trials


As a result of aforementioned trials, the following conclusions can be made.

  1. Ballistics. The ballistics of the 85 mm tank gun is nearly identical to that of the 85 mm AA gun model 1939, which makes it easy to use existing ammunition, artillery tables, and exploring the issue of armour penetration. If desired, the ballistics may be improved by mounting a muzzle brake, which will introduce the ability to improve penetration and lighten the load on the recoil system at the cost of abandoning shell compatibility with the 85 mm AA gun. Composing new artillery tables will not be difficult.
    Trials used shells from the 85 mm AA gun model 1939.
    Decrease in ballistics during the trials was negligible (near 1%) and can be traced to the condition of the barrel and chamber size changes (2-4 mm), which is typical for this kind of weapon.
  2. Precision. The precision of the 85 mm tank guns is very satisfactory at medium and close distances, nearly identical for both systems. However, the D-5T-85 system was more stable, due to the superior design of the recoil mechanism (positioned above), lesser vibrations, and less deviation when shooting.
  3. Range. The range is acceptable given the maximum elevation of the D-5T-85 on both tanks and S-31 on the IS: 12-13 km. For the S-31 gun on the KV-1S with a regular turret ring, this range is reduced to 7 km due to a limited elevation.
  4. Firing on the move trials shows that using these guns on the move is effective. At 10-15 kph, the guns hit the target 50% of the time and can be increased to up to 75%.
  5. The armour penetration of the guns is the same as the stock 85 mm AA gun: at point blank, the gun penetrates 98 mm of armour at 30 degrees, at 600-1000 meters, it penetrates 88-83 mm with regular AP-HE. Improved penetration may be achieved with the use of a muzzle brake, as stated above, or by using a subcaliber shell.
  6. Rate of fire. When firing a 16.2 kg shell from new caliber guns, the rate of fire is 10-12 RPM. Without adjusting aim, and with other favourable conditions, the rate of fire increases to 15 RPM. In old KV-1S turrets, this rate of fire cannot be reached due to more cramped working conditions, and is reduced to 5-6 RPM.
    The D-5T-85 system achieves a superior rate of fire due to its shorter recoil and smaller dimensions, which create more favourable working conditions for the crew.
  7. Robustness. The robustness is satisfactory, and use of stock 85 mm AA gun shells is acceptable.
    After previous trials, both guns have been improved, but the D-5T-85 gun has received more noticeable improvements. The recoil mechanism used to be weak, now it works acceptably and matches calculations. Recoil length is stable, there have been no failures to return.
    The S-31 system has demonstrated failures to return. Performance data differs from calculations, and the same breakdown that was seen earlier has occurred, where the bolts holding the mount cover broke off.
    The following defects occurred with both systems: 
    1. Insufficiently reliable breech. Misfires, accidental closures, shell rim biting, failure to extract.
    2. Unsatisfactory function of the trigger mechanism.
    3. Sharp, uneven return with a knock at the end.
  8. As mentioned above, the ease of use and service is greater in new IS turrets with the D-5T-85 gun as opposed to old KV-1S turrets due to the larger size of the turret and turret ring and smaller dimensions of the gun with a smaller recoil length.
    However, even in the IS turret, a series of defects need to be resolved, such as revising the ammunition rack, shields, and aiming, sight, and firing mechanisms. 
  9. TsAKB moved from 0-75 steel for the barrel to 0-70 steel. Parts quality needs to be considered carefully, considering unfortunate experiences with insufficient reliability of the S-31 barrel and breech. 
  10. The preparation of the vehicles and service during trials by factory #100 has done well. The S-31 system was prepared less thoroughly.

Based on trials held on August 22nd through 24th, 1943, of the 85 mm tank guns in the IS and KV-1S tanks, the Gorohovets proving grounds makes the following conclusions:
  1. The 85 mm tank guns are identical in their design, purpose, and characteristics compared to SPG guns tested on August 2-4, 1943 (GANIOP report #02913, August 6th, 1943). The guns have been improved and completed trials with fewer faults.
  2. The D-5T-85 is a superior design, which has the following advantages over the S-31:
    1. Lack of dead balancing weights in the turret.
    2. Smaller dimensions.
    3. Smaller (and acceptable) range of deviations.
    4. Greater robustness and reliability.
    5. Easier maintenance and assembly.
Because of this, the D-5T-85 gun designed by factory #9 can be recommended for use in heavy and medium tanks in the Red Army.

It is necessary to accelerate work on resolving the defects listed in this report."


  1. How would a muzzle brake improve penetration, and why would it make the shells incompatible??

  2. How would a muzzle brake improve penetration, and why would it make the shells incompatible??

    1. A muzzle brake would let you absorb more recoil, making it possible to fire the shell faster. Since the AA gun wouldn't have a muzzle brake, its recoil system would not be able to use this ammunition.

    2. Ahh, gotcha. Wasn't considering the gun as an overall system. Thanks!

    3. Ahh, gotcha. Wasn't considering the gun as an overall system. Thanks!

    4. But the 85mm Model 1939 AA gun does have a muzzle brake.

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