The completed work on increasing the power of the self propelled D-5 gun is analogous to that done by factory #8 on the 85 mm AA gun. It is important to note that we started working on increasing the muzzle velocity to 900 m/s before the start of the war in 1941, when we designed and partially completed the U-15 gun system mounted on the M-30 mount. The system was designed to combat enemy tanks. As the issue was solved with other artillery systems,work on the U-15 was never completed.
Currently, the factory received instructions to continue on this path of increasing the power of the D-5 gun by lengthening its barrel and increasing the density of loading, with the condition of retaining the current shell.
The barrel, chamber, and the dimensions of the D-5 system were borrowed from the 85 mm AA gun, as was the ammunition it uses.
The barrels of the 85 mm AA gun, and therefore the D-5 gun, are characterized by a small chamber, about 19% from the overall distance travelled by the shell. Usually, this distance ranges from 20 to 25%. The increase of this gun's power could not be very large, since the weight of the shell could not increase very much, and the density of loading could only be increased from 0.62 to 0.76. The barrel was lengthened to 64 calibers. The muzzle velocity reached was 900 m/s, with the pressure in the barrel of almost 3000 kg/cm^2 and chamber size of 14.5%.
If for the AA gun, the barrel length can be increased further, but the use of a tank gun with a longer barrel is not convenient. A more reasonable modernization of the D-5 gun would be the adoption of a longer one-piece round, the size of which would allow for increasing the gunpowder charge.
The D-5BM system increases the muzzle velocity by 108 m/s, or by 13.5%. It may be raised to 910 m/s by increasing the pressure slightly, to 3000-3100 kg/cm^2. The robustness of the barrel and other mechanisms allow for this.
In this case, the increase of the shell velocity compared to the D-5 gun would be 118 m/s, or almost 15%. We expect the penetration to increase by the same amount, using Krupp's formula.
The D-5BM gun has the advantage of preserving ammunition capacity, due to the same size shell. The ammunition rack size would be decreased when using a more powerful shell of the same caliber. Another advantage is that the shell only becomes 0.5 kg heavier, which is insignificant, allowing the rate of fire of the system to be maintained. Both of these factors play a significant role.
Using a D-5BM shell in a D-5 system allows the increase of the muzzle velocity by 40-45 m/s, which will cause no problems with robustness.
A further rational change would be to increase the ballistics of these guns by changing to NG gunpowder, with 10% increase in power and nearly 5% increase in the shell's initial speed.
November 7th, 1944"
CAMD RF 81-12038-344