Occasionally, Stalin personally gave advice to weapons designers. Here's an example.
The first solution: swap the barrel of the old gun. This design decision lengthens the barrel by 690 mm. The new barrel, combined with an increased charge, provides a muzzle velocity of 515 m/s, with a 6.23 kg shell. According to calculations, at 500 meters, against armour with coefficient 2400 at a 30 degree angle, the shell penetration will be 39 mm, and maximum range 8500 meters. The increased power is possible due to the use of a muzzle brake.
The experimental prototype fired 200 proof and 115 normal shots. The results are satisfactory.
This solution provides immediate production of more powerful regimental guns based on existing guns, as the changes to the production process are insignificant. Furthermore, it is possible to replace the barrels on old regimental guns in the field.
The second solution is to use the blueprints we have already developed and began production of. This prototype has muzzle velocity of 550 m/s, and range of 9500 m. The new gun weighs about 100 kg less than the old one. The rate of fire is increased due to the use of a semi-automatic breech from the regimental gun. This gun is smaller than the old one.
The gun will also be simpler to produce than the old one. The re-barreled old gun and the new gun will use the same shell.
Please advise on urgent government trials of the first prototype of the re-barreled gun."
The "old gun" is the 76.2 mm model 1927 regimental gun. Here is a photograph of the modernized re-barreled gun:
"76.2 mm regimental cannon ZiS-21-P, view from the front"
And here is a photograph of another modernized version of the mod. 1927, developed in Leningrad.
39 mm, though? Yawn. Let's get serious here.
"To: Chair of the Artillery Committee AU, Major-General comrade Hohlov
Chair of the NKV Technical Council, comrade Satel
June 18th, 1942
Engineers of the NKVD's 4th Special Department produced a proposal for modernizing the 76 mm model 1927 regimental cannon by means of replacing its barrel with a 122 mm mono-block.
According the the calculations of the specialists, the modernized gun will greatly increase the effectiveness of the shells against pillboxes, enemy personnel, and tanks protected with up to 90 mm of armour.
I consider this proposal realizeable, and have given an order for calculations to be made and a technical project developed. I send you a copy of the explanatory memo, and would like to know your preliminary conclusion regarding the sensibility of this project.
Attachment: short explanatory memo on 3 pages.
Chief of the 4th NKVD Special Department, Senior Major of State Security, Kravchenko
Short Explanatory Memo on the Subject of Modernization of the 76 mm Regimental Gun model 1927 by Turning it into a 122 mm Mortar
Stock armour-piercing shells from the 76 mm gun can penetrate 25-30 mm of armour. HEAT shells, at the current time, can penetrate up to 60 mm of tank armour, by forming a breach of no more than 20 mm.
Using a 122 mm monoblock barrel, penetration of up to 90 mm will be achieved, with a 30 mm breach. In 60 mm armour, the shells will form a breach 50-60 mm on entry, 90-100 on exit (exploding a 107 mm shell on armour results in a 60 mm breach on entry and 80 mm on exit).
The above data shows that the regimental cannon's anti-armour capacity can be greatly raised if it turned into a 122 mm mortar.
A HEAT shell for this weapon would weigh 13.5 kilograms, with 3 kg of explosive filler. The range, with an elevation angle of 24 degrees and 30 minutes will be approximately 3.5 km, and 4.25 km with an angle of elevation of 45 degrees.
Alongside the anti-tank performance, performance against infantry and pillboxes will also grow. If the 76 mm shell had 5.8 kg of metal and 0.6 kg of explosives, the 122 mm shell has 10 kg of metal and up to 3 kg of explosives.
Modernizing the 76 mm gun into a 122 mm mortar requires only the replacement of its 76 mm monoblock barrel with a 122 mm monoblock barrel, with no changes to the recoil mechanisms or mount.
The center of gravity is preserved. The recoil mechanisms can take 350 kg (the previous weight was 304 kg). This gives the muzzle velocity of the 13.5 kg shell as 230 m/s.
Only the piston changes in the gun's breech. The front part of the piston will increase in diameter, but the part that attaches to the frame will not change. This allows the frame and associated breech components to remain the same. The length of the piston does not change, so the firing mechanism may also be retained.
The extractor should be shortened due to the increase in diameter of the brass. Due to this shortening, due to the significant decrease of weight and length of the shell (pressure of gases decreases to 500 kg/cm^2), the extractor shoulder is sufficient.
A single piece shell, which differs little in weight compared to the 76 mm shell, but is much shorter, should maintain existing rate of fire.
During the investigation of this proposal, a 107 mm variant was considered. With a 9.8 kg shell, the range at 24 degrees and 30 minutes is 4.75 kg, and 5.9 km with an angle of 45 degrees. Using stock 17 kg shells, the range is 2.2 and 3 km respectively. The penetration was experimentally determined to be 70 mm.
We decided on the 122 mm variant due to a higher penetration (90 mm) and better damage to space behind the armour, as well as higher explosive-fragmentation effect on enemy personnel and fortifications.
Sadly, it was not to be. The elevation of this gun would only be 24 degrees, and vastly insufficient for indirect fire.