Based on the trials of the 122 mm SU-122P SPG on the chassis of the 100 mm SU-100 SPG, armed with the D-25S gun, having driven it 238 km, the Red Banner Central Artillery Proving Grounds has established that:
- The gun parts and mechanisms perform well in winter, and therefore easier, conditions. Final evaluation may only be performed on very poor terrain, and in the summer.
- In winter conditions, the air-tightness of the return mechanism is adequate. The air pressure did not change.
- The sight alignment is satisfactory. The zero of the sight did not shift during trials.
- The tray-shaped cutout in the breech and sliding block are on the right, when the breech opens leftwards, make the loader's job, placed to the left of the gun, difficult and dangerous. It is necessary to turn the gun to make the tray-shaped cutout be on the opposite side. There is not enough room to put the loader on the right of the gun.
- The fighting compartment is cramped and does not allow for comfortable work by the crew. This, combined with the aforementioned problem with the tray-shaped cutout, render the SPG a temporary measure, which must be armed with an existing gun and use an existing SPG hull with the goal of rapid improvement in firepower.
- The calibration of the sight is inconvenient in battlefield conditions (no access to the stopper screw for the dial). The design must be changed to make the stopper screw and horizontal dial easily accessible.
Additional notes on the comfort of working in the fighting compartment
The crew of the 122 mm SPG D25-SU-122P consists of 4 crewmen: commander, gunner, loader, and mechanic-driver. A more detailed review of the fighting compartment exists as a GAKP report #0583 from February 10th, 1945.
The commander is placed to the right of the gun. His workspace is cramped, the commander cannot comfortably fit between the wall and the gun, especially when the gun is at maximum traverse.
The commander's seat is installed up against the wall, which does not allow to sit on it comfortably and stably. The commander slides off, especially during harsh bumps in motion. There is nowhere to brace his legs against, which does not result in a stable position. In order to remain in place, the commander is forced to place one leg on the floor and hold the other half-bent. There are no guard rails, and the commander must take great care to not fall off his seat or hit himself on the gun.
The observation devices are placed in the commander's cupola, and consist of a MK-4 device with 360 degree vision and five observation slits. These devices allow observation of the battlefield with a deadzone of 10-15 meters. In winter, the slits can be stuffed with snow, and the commander cannot see out of them.
The gunner is placed to the left of the gun. The gunner's seat flips up and can be adjusted vertically and horizontally on its carrier next to the gun. The seat does not let the gunner sit on it comfortably during motion, namely, the protruding part of the guard plate forces the gunner to tilt to the left, which means he must press his shoulder and arm against the guard plate and hand trigger. The elevation mechanism is insufficiently comfortable to use, as the right hand is pressing against the guard plate and it's hard to move. The driver's seat and its back impede comfortable leg placement. Adjusting the seat horizontally is inconvenient.
The forehead rest hits the side level when the barrel elevates.
The loader is placed behind the gunner, to the left of the gun. During travel, the loader's seat is comfortable, placed in the rear left corner. In battle, he works while standing.
The existing configuration with the breech opening to the left, the loader's job is difficult, uncomfortable, and dangerous:
- It's hard to place the shell on the tray and then ram it in.
- When loading the propellant, it's hard to find the chamber.
- When performing two-piece loading, the gunner must load the propellant in two steps:
- Push in the propellant gently, leaving it 50-100 mm short.
- Quickly push the propellant all the way in, during which the loader must put his entire right arm in the chamber opening and press his chest against the guard plate or the breech rear. When the breech closes, his arm is lifted to the right (further out), which quickly tires the loader.
- The placement of the breech lock is on the right, like on the tank that carries this gun, and not on the left, where the loader is in the SPG.
In order for proper working conditions to exist for the loader, the breech must be rotated 180 degrees so that the tray-shaped cutout is opposite of the loader.
The driver is placed in front of the gunner. The driver's workspace does not provide him with sufficient comfort. The driver has to sit hunched over and hits his head on the gun's turning mechanism if he straightens up. When the SPG is in motion, the driver hits against the turning mechanism, specifically when the gun breech is rotated all the way to the left. The position of the horizontal aiming mechanism should be changed.
The ammunition rack of the D25-SU-122P contains 26 shells. The placement of shells and propellant in the rack makes their retrieval inconvenient, and should be revised. During trials, there were two cases of shells falling out of the rear ammunition rack due to weak locks.
On the comfort of the fighting compartment as a whole:
The fighting compartment is uncomfortable for the crew. The fighting compartment must be widened in order to increase the level of comfort. It is reasonable to widen the casemate, ie. make the casemate as wide as the rest of the SPG, increasing the space in the fighting compartment, which will increase crew comfort, allow for a more convenient ammunition rack, and a larger amount of shells.
Chief of the 4th Department, Engineer-Colonel Ivanov
Trials Supervisor, Engineer-Captain Vyvoldin"