This is the untold story of Britain’s deadliest illegal gun. This single handgun came to be used in 11 shootings in the space of six years. After shootings, guns in the UK are often passed on, traded, rented or hidden away. That is how this gun travelled across a major city without being seized, and was used in multiple, unrelated shootings. The only thing that connects all of these crimes is the firearm itself. The police give every gun they know about a different number, so that they can be tracked and monitored depending on their own unique characteristics. This particular gun is a black CZ 75 semi-automatic pistol made in the Czech Republic. Investigators have called it Gun No. 6. Now, almost a decade after the firearm was last used, an in-depth BBC Two documentary has tracked its journey.