A spray nozzle assembly comprising: a nozzle body that defines a central liquid flow passageway having an upstream mounting and liquid inlet section for coupling to a selectively controllable pressurized liquid supply and a downstream liquid discharge section (30 b); wherein an annular orifice member being disposed within said nozzle body and having a central liquid flow passage in fluid communication with said nozzle body passageway; characterized by: said orifice member including an upstream cylindrical section and a downstream cylindrical smaller diameter hub and the central liquid flow passage thereof including a conical passage section within the hub having a frustoconical configuration and being inwardly tapered in a downstream direction at an angle of between 20 and 40 degrees communicating with a discharge orifice of the orifice member for accelerating the flow of liquid directed through the orifice member; said orifice member being made of a resilient and pressure responsive deformable material such that upon an increase in the pressure of the liquid supply from 137.895KPa - 413.685KPa (20-60 psi) said liquid flow passage and discharge orifice thereof can be altered to increase the rate through said orifice member and nozzle body by at least 75 percent; and upon interruption of the supply of pressurized liquid to the orifice member the orifice member liquid flow passage and discharge orifice return to their original shape. a nozzle body that defines a central liquid flow passageway having an upstream mounting and liquid inlet section for coupling to a selectively controllable pressurized liquid supply and a downstream liquid discharge section (30 b); wherein an annular orifice member being disposed within said nozzle body and having a central liquid flow passage in fluid communication with said nozzle body passageway; characterized by: said orifice member including an upstream cylindrical section and a downstream cylindrical smaller diameter hub and the central liquid flow passage thereof including a conical passage section within the hub having a frustoconical configuration and being inwardly tapered in a downstream direction at an angle of between 20 and 40 degrees communicating with a discharge orifice of the orifice member for accelerating the flow of liquid directed through the orifice member; said orifice member being made of a resilient and pressure responsive deformable material such that upon an increase in the pressure of the liquid supply from 137.895KPa - 413.685KPa (20-60 psi) said liquid flow passage and discharge orifice thereof can be altered to increase the rate through said orifice member and nozzle body by at least 75 percent; and upon interruption of the supply of pressurized liquid to the orifice member the orifice member liquid flow passage and discharge orifice return to their original shape. said orifice member including an upstream cylindrical section and a downstream cylindrical smaller diameter hub and the central liquid flow passage thereof including a conical passage section within the hub having a frustoconical configuration and being inwardly tapered in a downstream direction at an angle of between 20 and 40 degrees communicating with a discharge orifice of the orifice member for accelerating the flow of liquid directed through the orifice member; said orifice member being made of a resilient and pressure responsive deformable material such that upon an increase in the pressure of the liquid supply from 137.895KPa - 413.685KPa (20-60 psi) said liquid flow passage and discharge orifice thereof can be altered to increase the rate through said orifice member and nozzle body by at least 75 percent; and upon interruption of the supply of pressurized liquid to the orifice member the orifice member liquid flow passage and discharge orifice return to their original shape.