Patent Publication Number: US-6210253-B1

Title: Longitudinally finished shotgun barrel and method therefor

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to shotguns and more particularly concerns the finish of the bore of a shotgun barrel. 
     Shotgun barrel bores are now generally “hone” finished to an approximately Ra 15 to Ra 17 surface grade with resulting striations oriented in a generally rotational direction in the bore. Since the striations are transverse to the path of shot and gasses traveling in the barrel, the shotgun demonstrates greater recoil and lower muzzle velocity than could have been achieved if the striations were not present. 
     It is, therefore, an object of this invention to provide a shotgun barrel having a bore which is striated in a generally longitudinal direction along the barrel. Another object of this invention is to provide a shotgun barrel having a bore finish that minimizes recoil of the weapon. A further object of this invention is to provide a shotgun barrel having a bore finish that maximizes the muzzle velocity of the weapon. It is also an object of this invention to provide a method for polishing the bore of a shotgun barrel to a high surface grade in comparison to the finishes achieved by conventional “hone” finishing methods. Yet another object of this invention is to provide a method for polishing the bore of a shotgun barrel which results in minimizing non-longitudinal striations on the bore surface. And it is an object of this invention to provide a method for polishing the bore of a shotgun barrel which results in generally longitudinal striations on the bore surface. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a method is provided for refinishing the bore of a shotgun barrel by pumping a relatively soft abrasive media longitudinally through the bore, thus achieving a higher surface grade. Use of the method further results in the bore having striations oriented in a longitudinal direction along the barrel. Since the striations are along the path of the shot and gasses which travel in the barrel, significant reduction in the felt recoil and increase in the muzzle velocity of the weapon is achieved. 
     In practicing the method for finishing the shotgun barrel, the gunsmith swabs the bore to remove dust. The bore is inspected to detect obstructions and blemishes and then secured to a support structure in a horizontal orientation. The grade of the surface finish of the bore is determined. A polishing media pumping system is connected to the receiver end of the barrel and all holes through the barrel wall are blocked. A silicone matrix media impregnated with silicon carbide is pumped from the receiver end of the barrel through the bore and out the muzzle end of the barrel to polish the bore until the bore striations are generally longitudinal. The barrel is disconnected from the pumping system and the support structure and excess media cleaned out of the bore. All blocking materials are removed from the barrel and air blown through the bore to clean out residue. A liquid cleaner is applied to the bore to remove oils and the like and the bore is swabbed. The grade of the surface finish of the polished bore is determined. If necessary, the barrel is resecured to the support structure and the previous steps repeated as necessary until a satisfactory grade of surface finish is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a side elevation view of the muzzle end of a typical shotgun barrel; 
     FIG. 2 is a partial longitudinal, diametric cross-section taken along the line A—A of FIG. 1 before the bore has been finished in accordance with the method of the present invention; 
     FIG. 3 is a partial longitudinal, diametric cross-section taken along the line A—A of FIG. 1 after the bore has been finished to an intermediate surface grade in accordance with the method of the present invention; 
     FIG. 4 is a partial longitudinal, diametric cross-section taken along the line A—A of FIG. 1 after the bore has been finished to a final surface grade in accordance with the method of the present invention; 
     FIG. 5 is a graphic representation of the surface grade of the bore as shown in FIG. 2; 
     FIG. 6 is a graphic representation of the surface grade of the bore as shown in FIG. 3; 
     FIG. 7 is a graphic representation of the surface grade of the bore as shown in FIG. 4; and 
     FIG. 8 is a flow diagram illustrating the steps of method of the present invention. 
    
    
     While the invention will be described in connection with a preferred embodiment and method, it will be understood that it is not intended to limit the invention to that embodiment and method. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning first to FIG. 1, the outer surface of a typical shotgun barrel  10  is illustrated. The barrel  10  extends from the chamber end  11  to the muzzle end  13 . 
     The bore  15  of the barrel  10  in the typical condition as received from the factory or after refinishing by conventional “hone” methods, is illustrated in FIG.  2 . As shown, the manufacturing and/or refinishing process results in generally rotational striations  17  in the surface of the bore  15 . By “generally rotational,” it is meant that the streaks or furrows of the striation  17  are not necessarily parallel or straight, but are closer to perpendicular to the longitudinal axis of the bore  15  than parallel to it. Since these striations  17  are, for the most part, transverse to the path of the shot and gasses exiting the weapon, they tend to increase the recoil and decrease the muzzle velocity of the shotgun. The rotational striations  17  are the result of use of machining and polishing tools which operate in rotational fashion. A more highly polished surface on the bore  15  will result in finer striations  17 . However, the finest finishes achieved with known methods are in the order of Ra 13 to Ra 15 micro inches. 
     The present method avoids the use of rotational tools in polishing. A relatively soft abrasive media is pumped longitudinally through the bore  15  to provide generally longitudinal striations in the bore  15  which in turn facilitate the passage of shot and gasses through the bore  15 . Looking at FIG. 3, after the present method has been used for an interval of time, the rotational striations  17  are reduced in number and depth and longitudinal striations  19  begin to appear in the surface of the bore  15 . Looking at FIG. 4, as the method is continued for a longer interval, the rotational striations  17  are substantially removed and the surface of the bore  15  is characterized by generally longitudinal striations  19 . By generally longitudinal striations, it is meant that the streaks or furrows of the striations  19  are not necessarily absolutely parallel to each other or straight, but are closer to parallel to the longitudinal axis of the bore  15  than perpendicular to it. The surface grades Ra of the bore  15  at the stages of the method illustrated in FIGS. 2,  3  and  4  are generally illustrated in FIGS. 5,  6  and  7 , respectively. As shown, the factory or conventional refinishing grade Ra of the barrel  10  used was, as an example, 27 micro inches. At the intermediate stage of the present method in which both rotational and longitudinal striations  17  and  19  appear, the surface grade Ra has been improved to 17 micro inches. At the final stage exhibiting substantially only longitudinal striations  19 , the surface grade Ra has been improved to 6 micro inches. Thus, the bore  15  has been polished to a much finer finish than was typically achieved by old polishing methods and, furthermore, the direction of the striations has been radically and favorably altered. 
     In the practice of the method of the invention, as is illustrated in FIG.  8 . The bore is swabbed  21  to remove dust. The gunsmith inspects  23  the barrel, including the housing, back bore, thread and choke, for obstructions and/or blemishes, perhaps by use of fiber optics or borescope. The inspected barrel is secured  25  in a horizontal orientation, preferably on a table using a granite surface plate with padded V-blocks. The grade of the surface finish Ra of the bore is then determined  27 , perhaps by use of a profilometer which reads the height or depth of the ridges or valleys of the striations in micro inches. An Ra greater than 13 has, in accordance with previously known polishing standards, been deemed unsatisfactory. The outlet of a polishing media pumping system is inserted  29  into the receiver end of the barrel. Approximately {fraction (5/16)} inch penetration will satisfactorily align the barrel and the media pumping system. The barrel is then bolted  31  to the pumping system. All holes through the barrel wall are blocked  33 , preferably using nylon blocks and tape. A silicone matrix media impregnated with silicon carbide is pumped  35  from the receiver end of the barrel through the bore and out the muzzle end of the barrel. A low to medium viscosity grade of media such as LMV-220 by Extrude Home Corporation of Erwin, Pa. is preferred. The barrel is then disconnected  37  from the pumping system. Excess media is cleaned  39  out of the bore, preferably by use of a nylon probe. All blocking materials are removed  41  from the barrel. Air is blown  43  through the bore to clean out residue. A liquid cleaner is applied  45  to the bore to remove oils and the like. “Gun Scrub” works effectively for this purpose. The bore is swabbed  47  again. The grade Ra of the surface finish of the polished bore is determined  49 . As before, this can be done by use of a profilometer to read the height or depth of the ridges or valleys of the striations in micro inches. A surface grade Ra less than 8 is generally satisfactory. The barrel is resecured  51  in its horizontal orientation. The previous steps are repeated  53  as necessary until a satisfactory grade of surface finish is achieved. The gunsmith then cleans and oils the bore and returns the barrel with the surface grade data to the shotgun owner. 
     A typical shotgun barrel has a chamber diameter of approximately 0.80″, an average bore diameter of approximately 0.73″ and a muzzle diameter of approximately 0.69″. Since the barrel tapers narrowingly toward the muzzle, the media is compressed as it passes through the bore. If, in the preparatory steps prior to pumping  35 , the bore is adequately cleaned, the media can be recycled to its reservoir for reuse. 
     Finish grades in the order of Ra5 to Ra7 have been readily achieved using this method. This method has, for example, also resulted in increasing the muzzle velocity of a weapon from 1200 ft/sec prior to refinishing to over 1400 ft/sec after refinishing depending on the actual bore diameter, surface finish, ammunition and temperature. 
     Thus, it is apparent that there has been provided, in accordance with the invention, a shotgun barrel and method of polishing it that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art and in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit of the appended claims.