- The 85 mm SPG consists of a somewhat modified 85 mm tank gun on the modernized SU-35M chassis (SU-122M).
- The 85 mm anti-tank SPG is designed to destroy enemy heavy tanks (90-120 mm of armour) at medium distances (500-1000 meters). In some cases, the gun can be used to destroy enemy pillboxes and armoured machinegun caps.
- The SPG needs to have good mobility and off-road performance, high mobility of fire, and excellent visibility of the battlefield.
- Artillery requirements
- The 85 mm anti-tank gun fires shells from the 85 mm model 1939 AA gun. In addition to this, a new HE-fragmentation shell will be designed (with the same ballistics) and subcaliber shells.
- Gun characteristics:
- Caliber: 85 mm
- Shell mass: 9.2 kg
- Propellant mass (gunpowder 14/7): 2.48 kg
- Muzzle velocity: 800 m/s
- Maximum pressure: 2500 kg/cm^2
- The barrel and chamber are the same as the model 1939 AA gun.
- Vertical range: -5 to +25 degrees.
- Horizontal range: +/-10 degrees.
- Recoil length: 320-390 mm.
- The mount and recoil mechanisms are the same as the 76 mm gun model 1941, with some modifications.
- The 85 mm anti-tank gun must be similar to the tank gun (see requirements #2419) and only differ due to a lack of a coaxial machinegun.
The design consists of an 85 mm AA gun barrel on the 76 mm ZiS-5 tank gun mount.
- The overall design must have high compatibility of parts and assemblies with existing mass produced 76 mm tank guns and 122 mm D-11 howitzers, with some change in production tools.
- The barrel should be a single piece, preferably without a muzzle brake. If a muzzle brake is used, use the stock breech. The breech is vertical, sliding, based on the 76 mm gun breech, with the same firing and semi-automatic mechanisms.
- Rate of fire is 20 RPM, with practical rate of fire (including aim adjustments) of 10-12 RPM.
- The hand trigger mechanism (no more than 8 kg of force) should be included in the elevation mechanism flywheel. Maintain the possibility of using a solenoid type electrical firing mechanism.
Note: the use of mount and recoil mechanisms from the D-11 howitzer are allowed, the recoil length can be any value.
- Aiming mechanism:
- Elevation: use from the ZiS-5 (D-11)
- Turning: screw type, with the speed of at least 0.75 degrees per wheel rotation, required force of 3 kg (from full stop).
- The oscillating part of the gun should be naturally balanced.
- The gun should be able to fire directly and indirectly. For this, a sight from the regimental cannon mod. 1927 and a telescopic sight are used.
- Requirements from the chassis:
- The modernized hull of the SU-122M (SU-35M) is used. Note: a secondary possibility is the use of captured PzIII and PzIV chassis.
- The size and armour configuration are unchanged. The mass, due to the decreasing mass of the gun, should be 1-1.5 tons less.
- The crew should be composed of 5: commander, driver, gunner, two loaders.
- Install the following observation devices:
- Commander: PTK and periscopic sight.
- Other crewmen: periscopic sights.
The total sum of observation ranges should result in good observation when stationary or in motion.
- Dead zones should be no more than 5-7 meters, 3.5 meters for the driver.
- External communication is done using a 9R radio, internal using a TPU-3F radio.
- There should be a hatch in the roof for signal flags or rockets.
- The following is kept in the fighting compartment:
- 60 gun shells.
- 1500 PPSh rounds.
- 25 hand grenades.
- 5 anti-tank grenades.
- 2 PPSh
A mixed ammunition rack is allowed, using horizontal and vertical placed shells.
- Aside from ammunition and tools, the fighting and driving compartment should have room for a first aid kit, water and rations, the crew's personal belongings.
- For self-defense, the SPG needs ports capable of fitting a PPSh. The dead zone of these ports should be no more than 10 meters.
- Two hatches should exist for regular use by the crew: one for the commander and a two-part hatch in the top. An emergency hatch should be placed at the bottom of the hull.
- There will be three types of parts kits:
- Per-gun, carried with every SPG.
- Per-battery, for 4 guns.
- Per-regiment, for 12 guns.
Chief of the 16th department, Colonel Zhevanik
Assistant to the chief of the 16th department, Engineer-Major Solomonov
April 13th, 1943"