Edit: Going to upload a video tomorrow.
This is the beta version of the Sentry Turret we know and love. It is basically a regular turret in game with a prop_dynamic of a shell parented to it that doesn’t have collisions. They were created in the 1990’s however a ton of them were accidentally spilled into the Underground Beta Testing Area: Moddb.com
So, they are an added challenge to the hazards of the Underground. More on them soon.