The SU-101 prototype with the 100 mm D-10S gun came with a combat loadout of 35 rounds. While the horizontal gun traverse at 22.5° and the vertical elevation at 18° were entirely sufficient, the engine compartment limited the gun depression at a very meagre −2°, almost eliminating the vehicle's ability to go hull-down. The bigger 122 mm D-25S gun of the SU-102 prototype, with 28 rounds of ammunition, was even more limited in its gun traverse, not being able to depress the gun further than 0.24° with an elevation of 18.5° and horizontal traverse of 19°. The very limited gun depression is one of the fundamental drawbacks of a rear-mounted gun layout.