Technical field [0001] 　The present invention relates to a method for producing a rifle tube having a spiral ribs on the inner surface. Background technique [0002] 　The water wall tubes subcritical power boiler, boiling phenomenon occurs that water is changed to steam. Such water wall tube, rifle tube is utilized. Rifle tube has a spiral ribs on the inner surface. A plurality of ribs, compared to the steel pipe having no ribs, to increase the surface area of ​​the inner surface. Therefore, rifle tube is to increase the contact surface between the inner surface and the water, increasing the power generation efficiency of the boiler. [0003] 　A plurality of ribs further stirring the water in the tube, to a turbulent state. Therefore, the occurrence of film boiling is suppressed. The film boiling, when water flowing through the pipe is transformed into a gas vapor at the boiling point is heated, a phenomenon that film-like vapor phase is generated in the pipe inner surface. If film boiling occurs, tubes will be overheated to a high temperature exceeding the boiling point, in some cases 噴破 by overheating occurs. A plurality of ribs, suppressing the occurrence of film boiling, suppresses 噴破 by overheating. [0004] 　Recently thermal power boiler, the improvement of the combustion efficiency, CO 2 is strongly sought improved emissions and (reduced). For these improvements, it is necessary to high temperature and high pressure steam. To achieve the high temperature and high pressure steam, rifle tube high-Cr, high strength is required. [0005] 　WO 2009/081655 (Patent Document 1) discloses a method for producing a rifle tube. As disclosed in Patent Document 1, rifle tube is usually produced by the following method. First, to prepare the steel pipe. A plug having a plurality of helical grooves, rotatable about the axis of the plug, attached to the distal end of the mandrel. Inserting a plug attached to the mandrel in the steel tube. Using a die, the plug is carried out cold drawing process with respect to the inserted steel tube. Through the above steps, rifle tube is manufactured. CITATION Patent Literature [0006] Patent Document 1: International Publication No. WO 2009/081655 [0007] 　As described above, the shape of the inner surface of the rifle tube is complicated. Therefore, in the cold drawing process, there are cases where loads applied to the mandrel becomes excessively large. In this case, there is a case in which seizure is generated in the plug. When producing a rifle tube high strength in particular, galling is likely to occur. Summary of the Invention [0008] 　An object of the present invention, the occurrence of seizing due to cold drawing can be suppressed, it is to provide a method for producing a rifle tube. [0009] 　Method for producing a rifle tube according to the invention comprises a first helical rib on the inner surface, to produce a rifle tube having an outer diameter of less 34 mm. The manufacturing method comprises a step of preparing a steel pipe having the following tensile strength 600 MPa, and a plurality of helical grooves, and a plurality of second helical ribs disposed between the spiral grooves, each adjacent comprises using a plug which satisfies the equation (1) and (2), by carrying out cold drawn against steel, and a step of producing a rifle tube. 　0.08