ZiS-30

If it is a ZiS it's an AT gun. Judging from the looks of the barrel I'd say it's a 57mm ZiS-2 on a Komsomolets artillery tractor (okay, it's a Russian model in the end) and the whole combination seems to have been called 'ZiS-30.'

When Germany invaded the Soviet Union in 1941, the largely obsolete Soviet tank force was reduced to a fraction of its original size. As one of the desperate stop-gap measures to counter the German tank forces, T-20 "Komsomolets" tracked artillery tractors were converted to tank destroyers.
Straining the chassis to the maximum, the resulting vehicle, the ZiS-30 was armed with the 57 mm ZiS-2. The conversion was made at the Joseph Stalin Factory no. 92 (ZiS), and about 100 were build in total.
The 57 mm ZiS-2 was an excellent anti-tank gun. Its standard APCBC shot (BR-271) was almost equal to the German 7,5 cm Pak 39 L/48, and its APCR shot (BR-271P) was superior to the APCR shot of the Pak 39 up to 1000 metres. Its main weakness was its poor HE capability. A variant of the ZiS-2 was also used on the T-34-57 prototype. The ZiS-30 was marred by very thin armour, however. Few were in service by 1942, and none survived the war.

The reason I am certain it wasn't an AA gun is that if you look at the mounting and shield you will see that it is designed to elevate down and not up as would be required of an AA gun unless they wanted to shoot planes that were landed, essentially that gun is already at maximum elevation.