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.
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 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.