This is the Clarke CAT84 2-in-1 pistol grip needle scaler and hammer tool, it can also be called a needlegun scaler or needle gun.
 
Nominal air consumption: 4cfm.
Max speed: 4500 blows/minute.
One piece alloy steel barrel.
Includes 3 chisels, 1 taper punch & spring retainer.
Length of piston stroke: 40mm.