This is a Brocock Orion air revolver with cartridges, made in the late 1990s.
Traditionally air-guns work by pumping compressed air into a designated space within the weapon, usually a hollow butt or a cylinder around the barrel. However in 1989 the Brocock company of Birmingham was founded and it patented 'BACS' (Brocock Air Cartridge System). In this system it was the metal cartridge itself which was filled with pressurized air, not the gun. In most cases the cartridges contained a standard .22 air-gun pellet. The cartridges were reusable and charged by means of a hand pump. This did away with the weighty spring-and-piston system of earlier air pistols and allowed the gun to function just like a breech-loading firearm.
In the UK, air weapons are now subject to stringent legislation regarding their muzzle velocity (maximum 6 ft per pound for air pistols and 12 feet per pound for air rifles) and, in the case of cartridge weapons such as this, the ease with which they can be converted to fire conventional 'live' ammunition.