This paper model is a Combine Sentry Gun (Turret), based on the video game Half Life 2, the papercraft is created by Evgeniy Pelevin. Combine Sentry Guns, referred to by Overwatch as sterilizers and as turrets by Rebels, are fully autonomous tripodal pulse weapons used by the Combine to guard choke points and vital areas.
Turrets are one of many human technologies adapted by the Combine for their own use after the Seven Hour War. The guns are often deployed as static defenses, used to guard areas that are infrequently patrolled, or as a means to supplement a larger defense network. The guns are fitted with small handles and are light enough to pick up and place by hand. When not in use, they are generally stored in small forcefield-equipped lockers.
Combine Sentry Guns, however, have a number of key differences from the model used by the HECU in Half-Life. Mainly, their targeting systems are more sophisticated. The Combine Sentry Gun does not rely on laser tripwires, instead incorporating a visual sensor into its main body, with which it can immediately detect intruders in its sight line. Once activated, it will sound an alarm and, after a brief delay, will open fire. Upon losing sight of its target, the gun will scan the area ahead by moving its upper body back and forth, before returning to its inactive state if the target does not reappear after several seconds. This variety of gun also incorporates a limited A.I. which is somehow able to distinguish between Combine forces, humans and Xen aliens. Another key difference is that the Combine Sentry Gun has a narrower field of view and range of motion, whereas the HECU Sentry Gun is able to rotate and fire in nearly any direction.
An individual gun's programming can be accessed and modified remotely through a separate computer terminal when necessary. In Chapter 9a, Entanglement, Alyx Vance reprograms a number of Sentry Guns, allowing Gordon Freeman to set them up to protect himself against protracted Overwatch assaults.
The Sentry Gun is loaded with the Combine's ubiquitous pulse ammunition, of which it has an essentially limitless supply, although in Episode One, some ceiling-mounted variants with no ammunition remaining are encountered in a zombie infested underground. Sentry Guns cannot be destroyed, and must instead be knocked over, whether by explosives, gunfire, the Gravity Gun, or by getting close to them and knocking them over manually. Once knocked over, the Sentry Gun's targeting systems will malfunction and the turret will shoot wildly and sound its alarm for a few seconds before shutting down, much like the Aperture Science Sentry Turret.
Its unstable tripod base defect was only noticed after manufacturing. On the body the alphanumerics "V952" can be seen, also featured on other Combine devices.
You can download this paper craft model here: Half-Life 2 - Turret (Combine Sentry Gun) Free Paper Model Download