The Rhunnish SIU-13 tank destroyer is a direct derivative of the stop-gap tank destroyer SSU-12-125, a variant of the SSU-12 designed for anti-tank warfare. The SIU-13 was designed specifically as a tank destroyer, armed with a massive 125-mm cannon and nothing else. The turret is also placed much farther forward than on the SSU-12, which more evenly distributes the weight of the turret and magazine against the engine and transmission. In theory, the SIU-13 was meant to operate in partisan groups against enemy tank divisions, able to destroy most tanks with a single shot and also able to take quite a bit of punishment if they engaged the enemy. In practise, however, the SIU-13 was never used in large numbers because the heavy tanks it was specifically designed to take out were rarely encountered, and most medium tanks could be destroyed with a single shot from the TVS-12's 90-mm cannon.