It's done with a paintball gun, but could be adapted to other things, like a NERF gun perhaps.

http://www.tomsguide.com/us/Sentry-Gun-Pai...news-14679.html

If we go with the "must not be F or L availability" then we can still do it I think.

The Scarecrow System
Ferret Drone (early warning system and designed to scare away animals)
Mechanical Arm (Safe to assume that it has 180 degree mobility)
Light Crossbow (Max range of 120 meters, use rangefinder to ignore penalties for range)
Screamer head arrows (Scares away animals if they dont die)
Radar (Rating 6)
Limited Maneuverability (Becomes a smart firing platform, can only rotate)
Camera (smartgun and targeting assists)

It becomes a stationary platform with 12 modification slots and with the improved sensor array it can detect most incoming threats with the right sensors.