'Ar weapons cache' this is where the police armed response unit, hence 'AR' store their weapons. These firearms don't get stuck on a rail after being cleaned, rather put into a bag and sealed in a cabinet.
Supposedly when people break in, they use bolt cutters on the outer and inner doors to gain access to the building completely. From there in, they use industrial code breakers which are rather expensive from my understand to break into these cabinets and steals the bags of weapons.
Once they've stolen all they can carry and fended off the police or negotiated, I've heard the robbers get into their method of transport and go to the money laundering facility near the airport and swap the weapons for cold hard cash. The never get the weapons out the bag, nor do they keep the weapons for themselves. These are smart guys and they don't use what they steal.
Essentially break into the weapons cache with the bolt cutters and code breakers and then sell the bags for £50,000 each at the weapons cache. Before you ask, no you cannot swap them for usable firearms.