Abstract: A the high velocity flame jet apparatus includes a supersonic gun having a main combustion chamber having two parts, one of which is air cooled and the other of which is water cooled. The air cooled portion of the chamber is provided with radial air inlet holes to allow the cooling air to enter the chamber and stabilize combustion. The water cooled portion of the chamber is elongated and terminates in a bend. A powder sprayer is arranged axially in the bent end of the combustion chamber and a removable exit nozzle is coupled to the distal end of the combustion chamber. Different removable nozzles are used for abrasive treatment and coating treatment. The first portion of the combustion chamber is provided with an ignition device coupling for coupling to a removable igniter. The removable igniter is coupled to fuel and oxygen sources and supplies the ignition temperature to the combustion chamber when the apparatus is started. After the gun is ignited, the igniter is removed.