Tests of the German 37-mm anti-aircraft gun FlaK-18 in a tunnel designed for this.
The tunnel was built to test anti-aircraft and aircraft weapons, it had 400 meters in length, the atmospheric pressure in the firing zone could be reduced to a level corresponding to the pressure at an altitude of 20 km.
Source of FlaK-18 photo information:
1. www.dlr.de
Location: Braunschweig, Germany
Date: 1940