This papercraft is a HECU Ground Turret, based on the FPS game Black Mesa of the Half Life series, the paper modelwas created by Evgeniy Pelevin. The HECU Ground Turret, or Sentry, is an automated gun deployed by the military during the invasion of the Black Mesa Research Facility for unmanned defense against unwanted force or entry.
The Sentry is greenish in appearance, has 3 red dots on the top, is mounted on a tripod, and has a small automatic 360° rotating gun. Usually, there is a tripwire that is wired to the turret, but it can be activated in other ways.
When being triggered, the turret will emit a high-pitched whine and deploy its automatic gun, then proceed to shoot the trespassing organism regardless of whether it is human or alien until it is dead, or the turret is knocked down. If the assailant goes out of the turret's view or dies, then it will proceed to shut off, until any other organism comes into its view.
Except for its 360° rotating gun, the sentry doesn't move, so looking for a way around to avoid crossing its line of sight is a good move. Also, it is possible to damage the sentry with conventional weapons or even knocking it over with another physics object.
If the player manages to close enough to a turret to grab it with the use key, Gordon will pick it up and reprogram it so it shoots every living thing in front of it, including HECU Marines, enabling the player to use it as an improvised weapon.
You can download this game paper model here: Black Mesa - HECU Ground Turret Free Paper Model Download