A high velocity thermal spray gun includes a nozzle member injecting an annular flow of combustible mixture at high pressure into a constricted gas cap. An outer flow of air is injected adjacent to the gas cap wall. Powder is fed axially from the nozzle member into the combustion chamber. An annular inner flow of air is injected coaxially outwardly of the powder. The nozzle face has a shallow annular slot therein between the outer and inner gas injections. Combustion gas and oxygen flow through separate annular gas channels to mutually impinge at the inlet end of the slot for effectual mixing, so that combusting of the combustible mixture is rooted in the annular slot.