The gun is most likely a Swedish-designed Bofors 80 mm anti-aircraft gun, manufactured in Hungary by MAVAG, all set up for action somewhere in western Russia. The unit's ammunition itself had been hauled there by Paul Nagy in a sizeable military truck, as he describes in his autobiography published recently.

Hungarian AA Artillery Position, Russia, 1941

AA guns soon proved to be effective against ground forces as well.  Paul Nagy states to have disabled this Soviet light tank with his Bofors AA gun.

Disabled Soviet T-26 Tank by Magyar

A Hungarian crew, having motored deep into the USSR often under difficult circumstances, sets into position its anti-aircraft artillery piece.