The XREP fits inside a special shotgun shell casing. Unlike standard shells, the cases for XREP devices are transparent. Taser chose transparent shells to make it easier for officers to identify the correct shell before loading it into a shotgun.
Just like a normal shotgun shell, the XREP shell uses gunpowder as a propellant. The shotgun ejects the XREP casing just as it would any normal shotgun round. But instead of firing a slug or round of shot, the shotgun fires an electronic projectile weighing 3.4 grams (about .12 ounces) [source: Taser].
This projectile has two main sections. The nose of the projectile has four sharpened electrodes. These electrodes pierce the clothing and skin of the subject and serve as the main point of contact for the electric charge. Before impact, the nose and second stage of the projectile move as a single unit. A pair of Kevlar-coated wires tether the nose to the second half of the projectile.
The second half of the projectile contains the electronics that allow the XREP to transmit voltage to a target. This includes a battery, a transformer and a microprocessor that acts as both a trigger and a monitoring device. The battery stores the electricity the XREP uses upon deployment. The transformer's job is to convert the electricity from the battery into a higher voltage.
A transformer converts alternating current from one voltage to another through a series of coils wrapped around a core -- two wires coiled around an iron nail could be a simple transformer. As electricity travels through the first coil of wires around the core, it creates a magnetic field. The magnetic field induces an electric field, which causes electrons to travel through the second coil of wires. There are step-up transformers that increase the voltage from an incoming source of electricity or step-down transformers that decrease the voltage.
The reason the XREP needs a step-up transformer is to create enough voltage to induce NMI in the target. Too few volts and the subject won't be incapacitated. Too many, and the target could be killed. To keep the Taser XREP from becoming a lethal weapon, Taser limits the amount of current flowing through the system to a few milliamps.
The base has six electrodes that unfold from the body of the projectile upon impact with a target. To help stabilize flight, the base of the projectile also has three spring-loaded fins that deploy upon ejection from the shotgun.
Let's look at what happens when you fire the XREP.