In the United States military, a rubber duck or "rubber ducky" or "Blue Gun" refers to a fake training weapon, usually an M16 rifle, used in basic training. Trainees are issued rubber ducks before they have been properly trained to use actual rifles, in order to become familiar with the care, responsible handling, and added weight of an M16 during various activities, such as bayonet drills, water survival and marches. They are also used to train soldiers in various ceremonial practices that differ when soldiers are armed. For example, standing at attention requires a different stance and set of movements when the soldier has a rifle in-hand.
Some rubber ducks are made by filling and coating an actual decommissioned M16 rifle with rubber or plastic. Some are also made using decommissioned rifle parts, with rubber or plastic used for the other parts. Still others are made entirely of rubber or plastic that has been molded to resemble both the exact shape and weight of a rifle.
Rubber duck use is being phased out in some areas of the Armed Forces, namely in Air Force Basic Training. They were replaced with M16 replicas: metal models that resemble M16 rifles, including most internal parts, but that lack the ability to fire. The replicas allow soldiers to learn disassembly and reassembly of their rifles much earlier in their training.