Blueprint for Armageddon artillery crews

In Episode 1 of Blueprint for Armageddon Dan mentions the artillery crews of Germany’s largest guns having to cover their eyes with cotton wads and open their mouths while firing the guns from quite a distance away.

The imagery itself sounds positively apocalyptic. Has anyone ever found a source for this information? I’ve done a little research and haven’t found any images or primary sources documenting this and I’m curious where else it may have been mentioned.

Thank you for your time